Extreme Environment Habitable Space Design: A Case Study of Deep Underground Space

Abstract

The deterioration of the global climate and accelerated urbanization have led to intense pressure on surface space resources. As a strategic development field, deep underground space has become a crucial direction for alleviating human habitation pressure. However, current research on deep underground space mostly focuses on fields such as geology and medicine, while the design of habitable environments lacks interdisciplinary integration and systematic approaches. Taking deep underground space as the research object, this study first clarifies the interdisciplinary research context through bibliometric analysis. Then, combined with geological data (ground temperature, groundwater, and ground stress, etc.) from major cities in China, it defines the characteristics of the in situ environment and the characteristics of the development and utilization of deep underground space. By comparing the habitable design experiences of extreme environments, such as space stations, Moon habitats, and desert survival modules, the study extracts five categories of design elements: natural conditions, construction status, social economy, users, and existing resources. Ultimately, it establishes a demand-oriented, five-dimensional habitable design methodology covering in situ environment adaptation, living support, medical and health services, resilience and flexibility, and existing space renovation. This research clarifies the differentiated design strategies for hundred-meter-level and kilometer-level deep underground spaces, providing theoretical support for the scientific development of deep underground space and serving as a reference for habitable design in other extreme environments.

1. Introduction

The Sixth Assessment Report (AR6 Synthesis Report: Climate Change 2023) released by the Intergovernmental Panel on Climate Change (IPCC) in 2023 points out that the global average temperature has risen by 1.1 °C, and all regions of the world are facing unprecedented changes in the climate system, ranging from rising sea levels and frequent extreme weather events to the rapid melting of sea ice [1]. While the surface environment is deteriorating, the global urban population continues to grow, putting the living environment of urban residents under unprecedented challenges. The World Cities Report 2022: Envisaging the Future of Cities, released by the United Nations Human Settlements Programme (UN-Habitat) at the 11th World Urban Forum, indicates that by 2050, the proportion of the global urban population is expected to rise from 56% in 2021 to 68% [2]. Against the backdrop of continuous global population growth and accelerated urbanization, surface resources in cities are under unprecedented pressure, with increasingly prominent problems such as tight land supply, space congestion, and ecological degradation. At the same time, with the rapid development of information technology and biotechnology, humans have extended their exploration boundaries to extreme environments such as the deep sea, deep underground (DU), outer space, and polar regions, enabling humans to live in increasingly harsh environments [3]. As surface survival pressure continues to increase and technology advances further, to enhance the future well-being of humanity [4], habitability research has expanded to broader spatial fields (see Figure 1).

From a broad biological perspective, habitability is defined as the ability of an environment to support the activities of at least one known organism [5]. Technological habitability refers to transforming environments originally uninhabitable for life into ones that support life activities through human technical means. Against this backdrop, this study defines an extreme environment as a place where human life is unsustainable without professional training and technical equipment [6]. The unsustainability of extreme environments can be short-term or long-term from a design perspective. Architects have long focused on sustainability in spatial design. In 1963, Serge Chermayeff and Christopher Alexander took the lead in proposing this concept [7]; subsequently, Buckminster Fuller further elaborated on it in his 1969 book Operating Manual for Spaceship Earth; since then, as shown in

Table 1. Classification of existing habitable space designs for extreme environments.

Extreme Environments	Continuous Unsustainable	Space Design	References	Extreme Environments	Instantaneous Unsustainable	Space Design	References
Sea	Ocean Surface	Habitats	[8]	Sea	-	-	-
	Deep Sea	Submersible	[9]
Land	Desert	Survival Module	[10,11,12]	Land	Mining Disaster	Rescue Chamber	[13,14]
	Polar Regions	Settlements	[15]		Flood	Modular Building	[16]
	Plateau	Buildings	[17]		Chemical Spill	Survival Module	[18]
	Deep Underground	Laboratory	[19]		Epidemic	Fangcang Shelter	[20]
Air	Outer space	Space Station	[21,22]	Air	Astronaut Return	Reentry Capsule	[23]
	Moon	Habitats	[24,25,26]
	Mars	Habitats	[4,27,28]

, studies on habitable space design for extreme environments and sustainable development have gradually become more comprehensive and refined.

Underground spaces exhibit characteristics of stable temperature and humidity, high concealment, and low external interference [29]. Their comprehensive and safe utilization is a crucial approach to achieving the sustainable development of the habitable environment, and they also serve as non-habitable spaces widely transformed by humans at present [30,31,32]. The literature indicates that urban underground space (UUS) can additionally expand the usable space of the ground surface by 25% to 40% [33]; therefore, UUS has attracted increasing attention worldwide [34]. However, with the comprehensive development of UUS, shallow underground spaces have gradually become saturated in the urban central areas and transportation hubs of cities such as Shanghai, Singapore, and Tokyo [35]. Against the backdrop of most metropolises worldwide generally entering the era of inventory construction, large-scale aboveground construction is restricted. As a strategic spatial resource of cities, deep underground space (DUS) plays a vital role in addressing the aforementioned issues.

Moving towards DUS—an environment with greater complexity and extremity—has become a new trend in spatial exploration and the realization of human sustainable development [36,37]. Compared with aboveground and shallow underground spaces, DUS possesses unique advantages in development and utilization. Practical cases of transforming DUS into habitable spaces include the Derinkuyu Underground City (85 m underground), dating back to the 8th century BCE [38], and the Survival Condo in the United States (see Figure 2 and

Table A1. Design information for Survival Condo.

Sub-Item Title	Details	References
Project Name	Survival Condo	https://survivalcondo.com/details/ (accessed on 19 September 2025); https://www.theguardian.com/artanddesign/shortcuts/2014/nov/12/for-sale-luxury-apocalypse-proof-condo-in-missile-silo (accessed on 19 September 2025)
Geographical Location	Prairie in Kansas, USA
Spatial Form and Scale	Converted from a cylindrical rocket launch silo with an inner diameter of nearly 16 m. Aboveground: Only one monolithic dome is exposed and wrapped by an approximately 2.7 m thick epoxy-hardened concrete wall. Underground: There are 13 floors in total, each with an area of nearly 200 m2, and a burial depth of 53 m.
Protective Structure	The entrance is equipped with a main gate with a thickness of about 0.5 m and a weight of 7.3 t, followed by 3 layers of explosion-proof doors. The overall concrete protection system can resist extreme disasters such as solar flares, volcanic eruptions, terrorist attacks, and virus pandemics.
Functional Modules	- Residential Module: Seven floors of apartment-style suites, including types like “full floor (about 170 m2)” and “half floor (about 85 m2)”. They are equipped with full-spectrum LED lighting systems and smart home systems and simulate outdoor scenery through video screens. - Public Service Module: Includes grocery stores, indoor swimming pool and spa, gym, cinema, library/classroom, bar, small prison, etc. - Support Module: Underground organic hydroponic farm, first-aid center, command and control center. Equipped with a water supply system with about 274 m3 water storage tanks, air filtration system, battery backup system, independent power supply system powered by wind turbines + diesel generators, and emergency food reserves that can last for several years.
Livability Support Systems	- Environmental Adaptation: Makes use of the relatively stable thermal environment of deep underground space and cooperates with the air filtration system to ensure indoor air quality. Alleviates the psychological and physiological discomfort caused by “no natural window scenery” underground through full-spectrum LED lighting and virtual scenery screens. - Resource Self-Sufficiency: The hydroponic farm realizes the self-production of vegetables and other foods, and the water storage, energy storage (batteries + independent power), and grain storage systems ensure the long-term independent supply of water, energy, and food, breaking away from dependence on aboveground resources.
Design Concept	Physical safety protection (disaster-resistant concrete structure and explosion-proof doors), psychological design considerations (lighting simulation and virtual scenery), application of advanced technologies (smart home, independent energy system), and the concept of a “big family” with diverse backgrounds (creating a sense of community in public spaces).

)—transformed from mid-20th-century missile silos with a buried depth of 53 m [39]. Looking to the future, some scholars have attempted to construct a technical system for the habitable environment in DUS [40] and have conducted vertical division of spaces from 0 to 2000 m underground, proposing a concept of hierarchical utilization [41].

However, in general, the current research focus on DU remains primarily concentrated in fields such as chemistry, geology, and medicine, while discussions in the field of architecture are extremely limited and all remain in the conceptual design stage [40]. In the field of engineering technology, engineering cases of DUS mainly include deep underground tunnels, deep underground laboratories, and deep underground metro systems; in the contemporary era, there are almost no engineering practices for long-term human habitation and residence [42]. Nevertheless, it is essential to conduct explorations into theories and preliminary design methods for DUS habitability design. This study first employs bibliometric analysis and, based on research findings from other fields, summarizes and analyzes the characteristics of DUS and reviews the design elements of habitable environments in DUS. Second, by comparing these elements with the design elements of human habitable spaces, such as habitats in other extreme environments, it extracts the design requirements for habitats in DUS. Finally, under the dual guidance of human-centered concepts and the concept of sustainable development, it proposes a preliminary methodology for habitable environment design in DUS. Given the irreversibility of underground space development, the early stage of constructing habitable DUS projects requires abundant accumulation of theories and design methods. To address the aforementioned research gaps—especially the limited architectural research on DUS habitability, the disconnection between interdisciplinary findings and design practice, and the lack of scale-differentiated design methods—this study explicitly proposes three core research questions to guide the subsequent investigation:

• From an architectural perspective, how can we define the depth boundary of DUS (integrating topographic differences) and quantify its extreme in situ environmental characteristics (e.g., temperature, humidity, in situ stress) based on geological data, which serve as the basis for habitability design?
• How can we extract and organize a core habitability design element system exclusive to the architectural field by synthesizing interdisciplinary DUS research (e.g., geology, medicine) and comparing habitability experiences from other extreme environments (e.g., space stations, lunar habitats)?
• How can we construct a demand-oriented DUS habitability design methodology that accounts for scale differences (100 m vs. 1000 m levels) to fill the research gap in architectural design for long-term deep underground habitation?

This research aims to, based on the existing accumulation of multi-disciplinary DU research, systematically sort out the extreme environmental characteristics of DUS, analyze the habitability design elements of DUS, and preliminarily establish a set of habitability design methodology systems. It is intended to provide a theoretical foundation for future habitable DUS construction projects and serve as a reference for the design of habitable spaces in other extreme environments.

2. Materials and Methods

2.1. Research Framework

This research framework proceeds as follows: clarifying the definition of DUS and analyzing DU characteristics via CiteSpace-based interdisciplinary research; extracting habitable experiences from typical extreme environments (outer space, etc.) at the macro/micro-scales; and finally, refining DUS habitability design elements and building a scale-differentiated (100 m, 1000 m) design methodology system (Figure 3).

2.2. Study Area

Based on the current global experience in DUS development, the development and utilization modes of DUS vary significantly across different regional environments. Cities in China, as well as countries such as the United States, Russia, Singapore, Finland, and Japan, all have different DUS division methods [43]. Japan began exploring urban DUS as early as the 1980s [44] and has accumulated extensive practical experience in DUS design. By the end of 2023, the annual newly added construction area of urban underground space (UUS) in China reached approximately 312 million square meters, with a year-on-year increase of 17.88%, making China a major country in global UUS development [45]. As shown in Figure 4 and Figure 5, for the convenience of this research, by sorting out the definitions of DUS from the Ministry of Land, Infrastructure, Transport, and Tourism (MLIT) of Japan and various cities in China, this study defines the depth boundary of DUS as 30 m below the ground surface.

The method of dividing underground space based on buried depth is not applicable in mountainous or sloped areas; instead, such space should be divided by regions based on topographic differences [46]. Specifically, areas with a ground slope of less than 8% can be regarded as flat areas, and 60° can be used as the boundary for dividing steep slopes and gentle slopes [47,48]. As shown in Figure 6, under terrains with different slopes, the development of underground space should involve hierarchical division along appropriate directions to optimize the definition and development of DUS.

2.3. Knowledge Graph of Deep Underground Space

Design technologies often exhibit a lag characteristic [49]. Research on DUS in the field of architectural planning needs to draw on existing interdisciplinary research findings. Since the relevant research involves a wide range of disciplines, it is necessary to use bibliometric analysis to clarify the dynamic research topics and evolutionary paths of DUS across various disciplines, explore the development patterns of DU research, and infer future development directions that can be referenced by the architectural planning field.

This study uses the CiteSpace software (Version 6.4.R1) for bibliometric analysis, with a detailed methodology to ensure reproducibility and rigor, as follows:

• Data Source: All literature data are derived from the Web of Science Core Collection database, which integrates the SCI-E (Science Citation Index Expanded) and SSCI (Social Sciences Citation Index) databases. This database is selected for its global academic coverage, high-quality peer-reviewed studies, and comprehensive inclusion of interdisciplinary DUS research (e.g., geology, medicine, architecture), ensuring the representativeness and reliability of the analysis sample.
• Data Acquisition Process:
  • Step 1: Initial Retrieval. The time frame is set as 1992–2025, with the search formula defined as follows: TH = (“deep underground” OR “deep-underground” OR “deep earth”). This initial retrieval yielded 4215 literature records.
  • Step 2: Primary Screening. Invalid non-academic records were excluded, including conference abstracts, journal announcements, calls for papers, and news reports, resulting in 3811 candidate records.
  • Step 3: Secondary Screening. This round excluded 103 records unrelated to “deep underground space” (e.g., studies focusing solely on “shallow underground”), ultimately retaining 3708 valid research records.
• CiteSpace Parameter Settings:
  • Clustering Algorithm: The Log-Likelihood Ratio (LLR) algorithm was adopted, which is superior for distinguishing semantically independent research clusters in interdisciplinary research, avoiding overlapping or ambiguous cluster labels [50].
  • Keyword Threshold: A “keyword co-occurrence intensity ≥ 20 times” threshold was used for secondary screening—this ensures that only high-frequency, influential keywords are retained, reducing noise from sporadic or marginal research topics.
  • Visualization Configuration: In the knowledge graph, node size reflects the citation frequency of individual studies (larger nodes indicate higher academic impact); line color represents the time of correlation between studies (enabling observation of research evolution trends); and cluster labels are generated by CiteSpace based on high-frequency keywords within each cluster (e.g., “deep underground geology”, “extreme environment habitability”), clearly identifying core research fields.

Through the above settings, 50 core keyword clusters (“temperature”, “high pressure”, “stability”, etc.) were finally retained, providing a clear mapping of interdisciplinary DUS research topics for subsequent architectural design analysis.

2.4. Relevant Data of Deep Underground Space

Currently, academic research on the development and utilization of DUS mainly focuses on fields such as mineral mining, energy storage, and DU medicine. It has reached a preliminary conclusion that DUS exhibits the environmental characteristics of “three absences” (absence of cosmic rays, sunlight, and oxygen) and “three highs” (high temperature, high pressure, and high humidity) [51]. The rock and soil mass in DUS is relatively hard and stable, with constant temperature and humidity, clean air, high CO₂ content, rich concentrations of negative oxygen ions, and high radon-derived radiation [52,53]. Additionally, extreme environmental conditions also affect human physiology, pathology, and psychology [40]. The aforementioned research has laid the foundation for the design of habitable environments in DUS. To systematically elaborate on the environmental characteristics of DUS, this study takes cities in China as the main research samples. It systematically describes the in situ environment of DUS regarding aspects such as the composition and structure of deep underground rock and soil, temperature and humidity, groundwater, vibration, and stress, clarifying the influencing factors for the development and utilization of habitable spaces in extreme DU environments.

The basic data on the rock and soil mass of Chinese cities is sourced from the GeoCloud Platform (https://geocloudsso.cgs.gov.cn/) (accessed on 26 April 2025); the underground temperature data of each city is from the Institute of Hydrogeology and Environmental Geology, Chinese Academy of Geological Sciences [54]; the data on the groundwater level burial depth, aquifer media, and burial conditions of each city is from the Yearbook of Groundwater Level Monitoring in China’s Geological Environment 2021 [55]; the data on CO₂ concentrations, atmospheric pressure, and radiation dose rates are derived from a baseline survey of environmental parameters in deep underground mines, led by a team of Chinese academics [53,56,57]. Additionally, this study also discusses the vibration and stress characteristics of DUS. The seismic vibration data are from a statistical study conducted in Japan in the early 21st century on earthquakes with a moment magnitude of 6.5 or higher [58]; the in situ stress of the surrounding rock mass is calculated using the following regression equation [59]:

$${\sigma }_V=0.0245H$$

(1)

$$\lambda ={\displaystyle \frac{129.58}{H}}+0.606$$

(2)

$${\sigma }_H=\lambda {\sigma }_V$$

(3)

where σ_v is the vertical in situ stress, with units of MPa; H is the cavern burial depth, with units of m; σ_H is the horizontal in situ stress, with units of MPa; λ is the lateral pressure coefficient, which represents the ratio of horizontal in situ stress to vertical in situ stress.

The in situ environmental characteristics of DUS constitute the design prerequisite for its development and utilization. However, the feasibility, direction, and effectiveness of its habitability design still need to be comprehensively evaluated within a more systematic framework. Currently, existing design element frameworks for underground space are primarily oriented toward shallow underground space [60], failing to cover the characteristics of DUS and the requirements of strategic scenarios, thus exhibiting significant limitations. Based on the DUS characteristics outlined above (Section 3.2 and Section 4.1), this study targetedly adjusts the existing design element framework, adding or revising key points distinct from those for shallow underground space—including responses to the physical and chemical environment deep underground, psychophysiological impacts, and deep underground resource development. It further summarizes a multi-dimensional coupling framework encompassing natural conditions [61], construction status [62], socio-economic factors [63], users [53,57], and existing resources [29]. These factors collectively constitute the key influencing variables for DUS development and utilization, and they are uniquely applicable to DUS. This part of the content will be elaborated on in Section 4.

2.5. Comparative Study with Other Extreme Environments

It seems challenging to build a habitable space design framework for the extreme DUS environment completely from scratch. Fortunately, the academic community has already conducted research on the habitability of other extreme environments, such as deep-sea and deep space, and relevant findings can provide indirect references for the habitability design of DUS. In the deep sea field, research initially focused on the design of manned submersibles [9] and subsequently expanded to technical frameworks for marine habitats, covering key systems such as energy supply, food security, air circulation, freshwater acquisition, and waste disposal. Some of these concepts are still in the conceptual exploration stage [8]. In the deep space field, research centers on Moon and Martian habitats. It not only defines the core principles of base construction and technical key points for self-sustaining operation [24,25] but also establishes a complete design framework ranging from site selection and planning to testing and verification [26]. Meanwhile, in-depth discussions have been conducted on directions such as light environment adaptation, multi-subsystem integration, and long-term sustainable operation [4,27,28,64], forming a relatively comprehensive research system.

In subsequent discussions, this study will first organize the core content of habitable space design research for other extreme environments and then focus on DUS to summarize and organize its specific habitable space design requirements. First, we will sort out the design logic of habitat systems at the macro-level and extract transferable experiences in spatial layout, life support, and safety protection. Then, we will analyze the environmental regulation, functional integration, and human adaptation mechanisms of small-scale units, such as space station modules, and summarize the key points of refined design. Finally, combining DU usage scenarios, we will identify the specific needs of users across multiple dimensions, with the aim of providing guidance for future designs.

3. Results

3.1. Results of Bibliometric Analysis

An analysis of keywords related to multi-disciplinary DU research is presented in Figure 7. Currently, the academic community’s research on DUS covers a wide range of disciplinary directions, and theoretical research fields such as deep underground physics, deep underground chemistry, and deep underground geology have reached a certain scale of research. However, there is still a lack of systematic research on the habitability of DUS. Based on existing findings, a brief review can be conducted from five perspectives, namely, DU spatial planning and layout, the physical and chemical environment, human psychological and physiological aspects, resources and energy, and safety and disaster prevention.

In terms of DU spatial planning and layout, as early as the 1980s, Japanese scholars had already conducted explorations on the development of DUS and infrastructure layout, and the current stratification range has been extended to below −2000 m [65,66,67]. Regarding the physical and chemical environment, although DUS has the advantages of low background radiation, ultra-quietness, and ultra-cleanliness, confined environments are prone to radon accumulation. Moreover, as depth increases, atmospheric pressure, temperature, humidity, and CO₂ concentration rise, while adverse factors such as dampness and heat intensify [41,68]. Research on human psychological and physiological aspects originated from studies on shallow underground and windowless spaces. Confined environments affect human circadian rhythms, and long-term DU work may induce issues such as fever and fatigue [69,70,71]. In the field of resources and energy, research on DU energy storage has reached a certain scale, and concepts of stratified underground integrated energy systems have emerged [72,73]. The safety and disaster prevention field focuses on fire and mine disaster assessment and evacuation, including the establishment of geological environment resilience models and the planning of evacuation routes [74,75].

Overall, preliminary outcomes have been achieved in DUS-related research across various fields, but these results have not yet been fully integrated. The following sections will systematically introduce the in situ occurrence characteristics of DUS, such as its compositional structure, temperature and humidity, vibration, and stress, and summarize and analyze its development and utilization characteristics. This aims to lay the foundation for the systematic summary of influencing factors for habitability design in extreme DUS environments in the subsequent sections.

3.2. Characteristics of Deep Underground Space

The solid substances in DUS are predominantly composed of rock and soil mass and also include cements, secondary minerals, and organic matter in the pores or fractures of the rock and soil mass. There are significant differences in geographical and geological environments worldwide, and geological structural characteristics have no uniform patterns. Taking China as an example, the classification results for rock and soil mass in representative cities are presented in

Table A2. Subsurface rock and soil mass composition and structure of major cities in China.

Landform	City	Depth * (m)	Rock Mass Type	Soil Mass Type (From Top to Bottom)	Structure
River Confluences	Wuhan	50	Quartz Sandstone, Limestone, Shale, Siliceous Rock	Mucky Soil, Cohesive Soil	Single-Layer
	Nanchang	370	Quartz Sandstone, Limestone, Shale, Siliceous Rock	Mucky Soil, Cohesive Soil	Three-Layer
Inland River Valley Terraces	Nanjing	43	Volcanic Rock, Limestone	Mucky Soft Soil	Single-Layer
	Changsha	30	Quartz Sandstone, Mudstone, Granite	Silty Clay	Single-Layer
	Nanning	50	Sandstone, Carbonate Rock, Siliceous Rock	Sandy Clay, Sand-Gravel	Double-Layer
	Lanzhou	200~300	Granite, Granodiorite	Loess, Silty Fine Sand, Gravel-Cobble	Three-Layer
	Lhasa	**	Granite, Volcanic Rock	Sand-Gravel, Sand-Gravel, Gravel-Bearing Silt, Sandy Clay	**
Low-Altitude Alluvial Plains	Guangzhou	200	Granite, Migmatite, Clastic Rock, Carbonate Rock	Marine Facies Silty Mud, Fluvial Facies Sand, Sand-Gravel	Three-Layer
Flat Areas of Plains or Basins	Xi’an	800	None	Loess	Single-Layer
	Zhengzhou	180	None	Silty Fine Sand	Single-Layer
	Hefei	60	Feldspathic Quartz Sandstone, Silty Mudstone, Thin–Medium Thick-Bedded Glutenite	Mucky Soil, Cohesive Soil, Silty Fine Sand, Silt	Double-Layer
	Jinan	30	Gabbro	Loess-like Silty Clay, Sand-Gravel	Double-Layer
	Shijiazhuang	60	None	Silty Fine Sand, Rubbly Soil	Double-Layer
	Taiyuan	500	None	Loess, Silty Clay Intercalated with Sand	Single-Layer
	Kunming	1000	Glutenite, Marl, Lignite, and Carbonaceous Claystone	Organic Clay, Red Clay	Single-Layer
Alluvial Fans in Front of Mountains	Chengdu	60	None	Fluvial Alluvial Cohesive Soil Layer, Silty Fine Sand, Sand-Gravel-Cobble Layer	Double-Layer
	Beijing	200	Migmatite, Marine Facies Clastic Rock, Carbonate Rock, Basalt	Cohesive Soil, Silty Sand Layer, Gravel-Cobble Layer	Three-Layer
	Urumqi	1100	Limestone, Sandstone, Shale, Oil Shale, Mudstone, Conglomerate	Fine-Grained Soil, Rubbly Soil, Gravel-Cobble Layer	Three-Layer
	Hohhot	**	Sandstone, Mudstone	Sand-Gravel, Clay	**
Mountainous and Hilly Areas	Guiyang	20	Sandstone Intercalated with Mud, Shale, Marl; Shale Intercalated with Mudstone, Dolomite	Red Clay, Secondary Clay	Single-Layer
	Chongqing	20	Mudstone, Quartz Sandstone	Cohesive Soil Layer	Double-Layer
Seasonal Frozen Soil Areas	Harbin	150	None	Silty Clay	Single-Layer
	Shenyang	30~80	None	Silty Clay, Medium-Coarse Sand, Gravel-Cobble	Three-Layer
	Changchun	20	None	Silty Clay, Medium-Coarse Sand	Double-Layer
Coastal Plains	Shanghai	200~300	None	Silt-Silty Clay, Mucky Clay	Single-Layer
	Hangzhou	200	None	Organic Clay, Sand Layer	Single-Layer
	Tianjin	300~500	None	Silt-Silty Clay, Mucky Clay	Single-Layer
	Haikou	500	None	Mucky Silty Clay, Silty Fine Sand (Containing Bioclasts)	Double-Layer
Coastal Mountainous Areas	Fuzhou	50~120	None	Mucky Soft Soil, Silty Clay	Single-Layer

Note: * Quaternary burial depth; ** no data available.

. Landforms are closely associated with tectonic activities, and geomorphic locations determine the type of surface rock and soil mass. In cities such as Hohhot, Fuzhou, and Wuhan, the rock mass is dominated by sedimentary rocks with a layered structure, hosting 80% of mineral resources, all fossil fuels, and most groundwater. The geology of the remaining cities is mainly composed of clay and loess, featuring an overall loose structure and good compressibility while exhibiting high porosity and strong permeability.

The liquid-phase substances in DUS include groundwater, brine, and hydrocarbon fluids. Groundwater is classified into phreatic water (unconfined) and confined groundwater. The groundwater level, aquifer media, and burial conditions of major cities in China are presented in Table A2. In most regions, the groundwater level is shallower than the depth of DUS, indicating abundant water content in these areas. Deep confined groundwater has poor mobility; though generally considered to have little impact on groundwater flow blockage, it still requires caution in engineering projects [76]. The brine contains high concentrations of ions and trace elements, and corrosiveness must be considered during its development. Hydrocarbon fluids are rich in organic matter, including crude oil and natural gas. Seepage flow can cause pore migration or even loss of soil particles. As soil pores expand, the seepage velocity increases, which may eventually lead to the formation of connected seepage channels within the soil mass, resulting in soil collapse. If DUS is developed in water-rich groundwater areas, not only is construction dewatering complex, but there is also a higher possibility of introducing new pollution sources due to local rises or falls in groundwater level—this may further lead to the swampification of some depressions or land subsidence. The high-seepage water pressure environment has a significant impact on the creep mechanical properties of brittle rocks deep underground.

The temperature of DUS is affected by solar radiation and the Earth’s internal heat, showing uneven distribution with increasing depth due to their combined effects [77]. Based on long-term geothermal observations, the vertical temperature of underground rock and soil (0–200 m) can be divided into three zones: the variable temperature zone (VTZ); the constant temperature zone (CTZ, with year-round constant CTZT); and the increasing temperature zone (ITZ). Underground temperature variations for representative Chinese cities are shown in

Table A3. Underground temperature and groundwater parameters of major cities in China.

City	Subsurface Temperature Parameters						Groundwater Parameters
	AWST 1 (°C)	CTZT (°C)	VTZ (m)	CTZ (m)	ITZ (m)	Geothermal Gradient (°C/100 m)	Aquifer Medium	Occurrence Conditions 2	Mean Burial Depth (m)
Harbin	−15.2/21.7	8.0	0~−36	−36~−54	<−54	3.3	Fissure, Pore	CW	−10.3
Changchun	−12.3/22.7	8.1	0~−30	−30~−40	<−40	2.7	Fissure, Pore	PW, CW	−7.0
Urumqi	−9.7/25.6	11.6	0~−30	−30~−40	<−40	1.7	Pore	PW, CW	−44.1
Shenyang	−7.7/23.8	9.7	0~−23	−23~−48	<−48	2.6	Pore	PW, CW	−4.0
Hohhot	−8.3/21.8	10.0	0~−38	−38~−48	<−48	2.5	Pore	PW, CW	−17.4
Xining	−6.0/16.1	11.3	0~−20	−20~−31	<−31	4.0	Pore	PW	−11.9
Beijing	−3.4/25.2	14.0	0~−15	−15~−45	<−45	3.0	Pore, Karst	PW, CW	−24.6
Tianjin	−1.1/26.7	13.5	0~−20	−20~−30	<−30	3.0	Pore	PW, CW	−16.6
Yinchuan	−3.5/24.0	12.5	0~−30	−30~−45	<−45	3.5	Pore	PW, CW	−9.7
Shijiazhuang	−0.7/25.9	15.0	0~−19	−19~−39	<−39	3.0	Pore, Karst	PW, CW	−44.2
Taiyuan	−2.4/24.1	13.0	0~−20	−20~−50	<−50	2.8	Pore	PW, CW	−20.9
Jinan	0.1/25.5	16.7	0~−17	−17~−35	<−35	2.2	Fissure, Pore, Karst	PW, CW	−16.
Lanzhou	−5.9/18.6	12.1	0~−24	−24~−33	<−33	3.0	Pore	PW	−18.2
Zhengzhou	3.4/27.4	16.0	0~−18	−18~−30	<−30	3.0	Pore	PW, CW	−28.6
Xi’an	2.4/25.6	16.0	0~−23	−23~−33	<−33	3.9	Pore	PW, CW	−20.4
Lhasa	−0.1/16.4	11.6	0~−33	−33~−64	<−64	2.5	Pore	PW	−11.0
Guiyang	5.3/27.9	18.0	0~−17	−17~−31	<−31	2.7	Karst	PW	−11.3
Kunming	5.0/27.7	17.4	0~−12	−12~−32	<−32	3.2	Fissure, Pore, Karst	PW, CW	−8.1
Nanjing	7.2/28.2	17.9	0~−13	−13~−23	<−23	3.0	Fissure, Pore, Karst	CW	−18.5
Hefei	6.3/28.6	18.3	0~−15	−15~−23	<−23	1.9	Pore	CW	−6.3
Shanghai	7.4/25.5	18.5	0~−10	−10~−39	<−39	1.5	Pore	PW, CW	−5.4
Wuhan	7.2/28.0	19.1	0~−15	−15~−30	<−30	2.5	Fissure, Pore, Karst	CW	−5.7
Chengdu	7.8/27.1	19.8	0~−11	−11~−41	<−41	2.0	Fissure, Pore	PW	−5.3
Hangzhou	8.0/28.7	19.6	0~−17	−17~−44	<−44	3.4	Pore	PW, CW	−6.5
Chongqing	7.4/28.6	19.5	0~−18	−18~−28	<−28	1.3	Fissure, Karst	PW, CW	−9.0
Nanchang	6.9/24.6	16.3	0~−30	−30~−40	<−40	2.0	Fissure, Pore	PW, CW	−6.5
Changsha	10.2/21.6	16.8	0~−25	−25~−40	<−40	3.0	Fissure, Pore	PW, CW	−5.3
Fuzhou	12.2/27.9	22.8	0~−15	−15~−50	<−50	3.1	Fissure, Pore	PW, CW	−4.6
Guangzhou	16.0/29.3	23.9	0~−12	−12~−28	<−28	1.8	Fissure, Pore, Karst	PW, CW	−4.9
Nanning	14.6/28.6	24.0	0~−18	−18~−23	<−23	3.9	Pore, Karst	PW, CW	−7.6
Haikou	19.4/29.1	27.5	0~−13	−13~−25	<−25	2.9	Fissure, Pore	PW, CW	−17.3

Note: 1. AWST is the 5-year average winter and summer temperatures on the ground surface; 2. PW is phreatic water; CW is confined water.

: the 5-year average winter–summer temperature difference on the ground is 10–30 °C, which disappears at the burial depth of the CTZ roof. Temperatures at −30~−50 m are basically constant, reflecting DUS’s constant temperature property [78]. In cold regions, the CTZ has a lower temperature, greater burial depth, and greater thickness. Temperatures in the DU at depths of −30 to −100 m are mostly 10 to 20 °C, significantly lower than the nearly 45 °C surface temperature variation. At −200 m, the temperature is 3~5 °C higher than the CTZ, mostly falling within the 15~25 °C comfortable range.

Analysis of DUS seismic acceleration shows that horizontal seismic acceleration amplitude changes mainly in shallow layers: it varies drastically within 0~−100 m, drops to 0.2 times the surface amplitude near −100 m, and stays relatively stable in layers deeper than −100 m. The variation characteristics of vertical acceleration amplitude are similar to those of horizontal acceleration, but its amplitude ratio is slightly larger than that of horizontal acceleration at the corresponding depth; see Figure 8a. This reflects the superior seismic resistance performance of DUS. The curve of in situ stress variation with depth is shown in Figure 8b. In underground spaces at depths exceeding 1000 m, high in situ stress is the most typical environmental occurrence characteristic of deep engineering, accompanied by new engineering geological phenomena such as rockbursts and core disking. During the excavation process of DUS, there may be a risk of gas outburst, and it is imperative to adopt necessary prevention and control measures during the construction process.

Data on the CO₂ concentration, relative humidity, atmospheric pressure, and total γ-radiation dose rate in DUS came from 2019–2020 environmental surveys by China National Gold Group’s Jiapigou Minerals Erdaogou Mine (CJEM) [56] and China Pingmei Shenma Group (CPSG) [57]. These results are presented in Figure 9. The CO₂ concentration in DUS remains relatively stable at depths shallower than −900 m but changes abruptly near −1000 m, increasing by 30%. Relative humidity rises with depth: approximately 80% at −1000 m in the 12th Mine of CPSG, while that in CJEM at −1000 m is nearly 100%. The atmospheric pressure in DUS increases with increasing depth, whereas the total γ-radiation dose rate in DUS decreases with increasing depth.

When designing habitable spaces in extreme DU environments, the unique in situ environment of DUS gives rise to a series of characteristics during development and utilization, such as closed and external isolation characteristics, uniqueness of construction and operation–maintenance, irreversibility of development and impacts, and continuity and correlation of resources.

Due to its deep burial depth and confinement by rock and soil mass, DUS exhibits significant closed and isolation properties. Advantages include stable internal environments, which are suitable for the layout of special facilities, such as flood control and storage systems and nuclear facilities; isolation of adverse impacts on surface ecosystems; resistance to the intrusion of radioactive or toxic substances; and protection against disasters like earthquakes and typhoons [79]. Disadvantages, however, include difficulties in smoke extraction and rescue during fires [80]; susceptibility to flood inundation; poor ventilation, leading to temperature and humidity imbalance; and pollutant accumulation. The construction costs of DUS are significantly higher than those of surface and shallow underground spaces, with long investment payback periods. Nevertheless, costs can be reduced through route optimization and the use of geological materials as construction materials. In terms of operation and maintenance, DUS dominated by clay and loess has low structural bearing capacity, while DUS dominated by sedimentary rocks presents high construction difficulty, though it offers high stability and low maintenance costs after completion. The continuity of DUS media, or the force balance with upper structures, is a necessary condition for the safety of surface buildings. The development of DUS tends to alter the rock and soil environment, resulting in difficulty in secondary planning, demolition, and reconstruction. Particulates, lighting, and other factors in the artificial environment of DUS can cause irreversible impacts on the surrounding geothermal environment, groundwater, biological communities, and rock–soil geological environment.

4. Discussion

4.1. Environmental Characteristics and Design Elements

To more intuitively present the environmental characteristics related to the habitability of deep underground spaces, this paper selects the Moon habitat environment and space station environment—areas where extreme environment habitability research is relatively mature—along with the Earth’s surface environment and environmental reference values, and compares them with deep underground space environments (at the hundred-meter and -kilometer scales), as shown in

Table 2. Comparative reference standards for environmental comfort for DUS, surface, outer space, and the Moon.

Category		1000 m Scale Underground	100 m Scale Underground	Surface	Outer Space	Moon	Environmental Comfort Reference Standards [81,82]
Living Space		−100 to −1000 m	−30 to −100 m	148 million km2	110 to 916 m3	Polar Lava Tubes	-
External Environment	Temperature	10~50 °C	8~30 °C	−90~50 °C	−270 °C	−190~137 °C	-
	Wind Speed	None	None	5 m/s	None	None	<5 m/s
	Gravity	Approximately 1 g	Approximately 1 g	1 g	None	1/6 g	-
	Atmospheric Pressure	Approximately 110 kPa	95 to 103 kPa	101.3 kPa at sea level	None	3 × 10−3 kPa	High O2 partial pressure: 34.5 to 103 kPa
	Circadian Rhythm	>24 h	>24 h	23 h 56 min	None	28 d (14 + 14)	-
Internal Environment	Gas Composition	500 ppm CO2	450 ppm CO2	419.3 ppm CO2; 21% O2; 78% N2	Artificial environment	Argon (Ar), neon (Ne), etc.	≤1000 ppm CO2
	Temperature	10 to 30 °C	10 to 20 °C	20 to 26 °C	20 to 27 °C	None	22 to 28 °C (summer); 16 to 24 °C
	Relative Humidity	40% to 80%	30% to 50%	30% to 60%	25% to 75%	-	40% to 80% (summer); 30% to 60% (winter)
	Illuminance	-	-	150~500 lx	200~750 lx	-	75~750 lx
In Situ Resources		Minerals, water resources, space resources, and special strategic resources	Minerals, water resources, and space resources	A wide variety of resources	None	Lunar soil, lunar rocks, etc.	-
Potential Energy Sources		Geothermal energy, fossil energy	Geothermal energy, fossil energy	Solar energy, wind energy, tidal energy, etc.	Solar energy (intermittent, limited)	Solar energy (continuous, abundant)	-

.

DUS exhibits relatively stable external temperatures. At depths of around one hundred meters, the air pressure remains within a comfortable range, whereas at depths of one thousand meters, high-pressure conditions are prevalent. The day–night rhythm in deep underground space is slightly longer than that on the surface. Regarding the internal environment, the in situ CO₂ concentration in deep underground space increases with depth but remains within standard limits. At depths of around one hundred meters, both temperature and humidity levels are suitable for habitation, whereas at depths of one thousand meters, high-temperature and high-humidity conditions are present. Natural light has difficulty reaching deep underground spaces, but there are still design approaches to indirectly introduce natural light. Studies and practices related to space stations and Moon habitats have emphasized the importance of in situ resource utilization, and deep underground space similarly possesses abundant materials and energy, presenting significant development potential. Based on the extreme environmental characteristics of the deep earth, the core variables for designing habitable spaces can be inferred. The design elements can be preliminarily categorized into five types: natural conditions, current construction status, socio-economic factors, user demographics, and existing resources [60].

4.1.1. Natural Condition Factors

Natural condition factors serve as the material foundation for development, covering core aspects such as the engineering geology of rock and soil mass, hydrogeological conditions, unfavorable geological structures, topography, geomorphology, and natural sensitivity (e.g., nature reserves). These factors directly determine the difficulty of development and safety risks.

4.1.2. Construction Status Factors

Construction status factors focus on practical development constraints. There are significant differences in the supporting levels of existing aboveground and underground infrastructure: suburban areas have stronger development momentum due to complete supporting facilities and financial advantages, while areas far from urban centers face higher development difficulty due to insufficient infrastructure. Additionally, the distribution and functions of existing projects (e.g., shallow underground spaces, surface buildings) also impose constraints on the vertical stratification and horizontal connectivity of DU development.

4.1.3. Socio-Economic Factors

Socio-economic factors provide support and guidance for development. From a socio-political perspective, China has identified “marching toward the deep Earth” as a key strategic scientific and technological issue that must be addressed. Hong Kong, China, has clarified the development direction of DUS through the special planning document Cavern Master Plan, responding to Hong Kong’s long-term development. Countries including Japan, Finland, and Singapore have also promoted DUS development through strategic planning—such as Finland’s Underground Master Plan of Helsinki [83]—forming a cross-border consensus on the political orientation for DUS development. From a regulatory perspective, Japan has strictly defined DUS and standardized the conditions and procedures for public project development through the Act on Special Measures concerning Public Use of Deep Underground [43]. Finland, Singapore, and other countries have also provided regulatory bases for DUS planning. From an economic perspective, land prices in urban central areas, transportation hubs, and other locations are high. DUS development, not restricted by surface location, holds a cost advantage in key planning areas [44]. DUS harbors resources such as geothermal energy, minerals, and energy storage capacity [68], boasting significant economic benefits. Additionally, utilizing geological materials in the development and construction of DUS can reduce construction costs, and the proceeds from resource utilization can offset DUS construction costs—further enhancing the economic feasibility of DUS development.

4.1.4. User Factors

User factors relate to the demand adaptability of development. From a psychological perspective, the silence and enclosed nature of DUS tend to trigger negative psychological perceptions, while environmental factors like temperature, humidity, and noise indirectly exacerbate emotional fluctuations [70]. Workers in deep underground gold mines often experience physical discomfort (e.g., fatigue, insomnia, frequent dreams) and psychological stress [56]. Additionally, DUS lacks natural light, and artificial lighting cannot replicate circadian rhythms; prolonged abnormal lighting disrupts the secretion of melatonin (a sleep-regulating hormone) and cortisol (a stress hormone) [84], further impairing occupants’ sleep quality and psychological stability. From the perspectives of ethics and long-term social adaptation, DUS development has irreversible impacts on the geological environment and resources. For example, at deep underground cultural heritage sites such as Slovenia’s Škocjan Caves, infrastructure, lighting, and air conditioning systems have permanently altered the heritage environment and microclimate [85]. Studies in China, Japan, and other countries show that DUS development causes hard-to-restore damage to geothermal energy, groundwater, and biological communities. Meanwhile, DUS development may conflict with shallow underground spaces and surface buildings; its enclosed nature also makes rescue and evacuation during fires or floods extremely difficult, threatening occupants’ lives. DUS design tends to have high spatial homogeneity (e.g., constant lighting and temperature), leading to wayfinding difficulties. Linguistically, terms like “deep” and “underground” are often linked to negative concepts; this may hinder occupants from developing a sense of community identity and building stable social interaction networks. Moreover, insufficient fresh food sources in DUS may further increase the difficulty of long-term adaptation.

4.1.5. Existing Resource Factors

Existing resource factors target differentiated development scenarios. For developments centered on abandoned mines and special mining pits, it is necessary to balance existing resource utilization, pollution control, and equipment reuse. Meanwhile, the high-risk nature of such spaces tends to reduce people’s willingness to participate, requiring targeted optimization of safety design and psychological guidance.

4.2. Existing Research on Habitability in Extreme Environments

As a typical extreme environment, DUS shares common characteristics with other extreme scenarios such as outer space, polar regions, and deep sea environments, including ecological isolation, resource constraints, and the need for human adaptability. The mature design experiences from space stations and polar research stations, such as modular construction and closed-loop life support systems, can provide technical parallels for DUS design. This section will review habitable design methods from other extreme environments, distilling their core strategies to offer interdisciplinary references for optimizing DUS design elements. Among various extreme environments, the design for habitability in space stations is relatively mature, with numerous international standards and technical manuals already published. Additionally, there is a rich body of theoretical research concerning habitats on the Moon and Mars.

Regarding the temporal scope selection criteria for comparative cases, priority is given to papers, reports, or completed engineering projects published between 2015 and 2025 while striving to ensure the diversity of cases. For habitats in the macro-dimension, emphasis is placed on long-term human habitation sustainability, as well as the integrity of system functions and the coordination and compatibility of modules. Therefore, case materials must include comprehensive content such as design responses to extreme environments, system composition, functional modules, and key habitability technologies. After prioritizing highly cited studies, three types of habitats are identified: Mars habitats, Moon habitats, and maritime habitats. For capsules in the micro-dimension, focus is placed on short-term human habitation sustainability, considering spatial efficiency, emergency response, human physiological and psychological adaptation, and internal compact circulation. Thus, case materials need to cover content including design responses to extreme environments, habitat creation, quantified key habitability data, and key habitability technologies. After prioritizing highly cited studies, three types of capsules are determined: space stations, desert survival capsules, and (mine-type) emergency shelter capsules.

4.2.1. Design of Habitats in Extreme Environments

Habitats on Mars and the Moon need to create a sealed environment to deal with challenges such as cosmic radiation, vacuum or thin air, extreme temperatures, and dust storms [86]. Martian habitats face issues like seawater corrosion and the impacts of ocean currents and hurricanes. However, extreme environments also offer design opportunities: the ocean is the only place on Earth where all forms of renewable energy are concentrated [8]; lunar regolith has low thermal conductivity, providing good insulation properties; and Martian soil can form sturdy materials with good interlocking properties [87]. Representative habitable design cases in extreme environments show that in situ resource utilization and standardized modular design are common features in the design of habitats in all extreme environments. Greenhouses, which provide both food and psychological comfort while participating in the ecological cycle, are the most basic functional module for human habitation (see Table 3).

As mentioned above, the focus of habitability design for habitats in extreme environments lies in addressing environmental constraints through functional space design; however, achieving a higher level of livability also requires the synergy of three dimensions—function, ecology, and culture. By systematically integrating environmental adaptability, functional performance, and cultural continuity, the adaptability of the long-term habitable functional system in a space and its sustainable development can be enhanced [88]. For DUS, this logic can be translated into the following: in different depth scenarios ranging from 100 to 1000 m, it is necessary not only to optimize structural safety and energy utilization based on environmental parameters, such as in situ stress and geothermal temperature, but also to incorporate adaptive designs that align with the surface’s regional spatial and cultural characteristics (e.g., the layout logic of public spaces, preferences for light and shadow perception). This helps avoid cultural alienation caused by the extremely enclosed environment. For instance, by simulating the regionalized spatial scales of the surface and optimizing the rhythm and color temperature of artificial lighting, the unity of functional practicality and humanistic adaptability of DUS can be achieved.

Table 3. Comparison of habitability designs for habitats in extreme environments. The listed extreme environments include Mars, the Moon, and the sea surface.

Design Dimensions	Mars Habitats	Moon Habitats	Maritime Habitats
Extreme Environments	Low gravity, extreme temperatures, strong radiation, dust storms, micrometeorites, Marsquakes	Microgravity, extreme temperature differences, strong radiation, low atmospheric density, lunar dust, Moonquakes	Tidal currents, seawater corrosion, hurricanes/typhoons, ocean current impact, dynamic fluid medium
Design Response	Variable modular units, glass curtain wall with virtual reality interface, building materials produced from in situ resources, power generation by nuclear fission reactors, temperature and humidity control, emergency shelter bunkers, habitat flow field control	Solar photovoltaic panels and thermoelectric power generation, building materials produced from in situ resources, detection and extraction of ice-water on the lunar surface, construction of permanent underground living areas in lunar lava tubes (with connected modular units), digital management	Semi-submersible structures, renewable energy utilization, energy generation and bioproduct production from marine algae, temperature and humidity control, modular design, combined constant–variable pressure structures, oxygen storage, stabilization and mooring
System Composition	Closed life support, energy supply under dusty conditions, air pressurization and supply, waste management, pollution monitoring system	Energy, communication, material supply, waste management, product return to Earth, system operation, self-circulating ecosystem	Energy, water-based food, air, closed-loop water and marine waste management, offshore ecology, circular biological operation system
Functional Modules	Greenhouse ecological park, mobile medical unit, isolation and shelter area, three-level power generation field of nuclear energy–solar energy–wind turbines, power storage device, parking lot, entertainment and social venue, work and laboratory, robot work area	Greenhouse planting base, small animal breeding base, domestic waste recycling facility, oxygen production laboratory, mineral extraction and storage area, biopharmaceutical laboratory, earth-moon relay station, public facilities for medical treatment, education, leisure, commerce and sports	Marine aquaculture base, ocean thermal energy conversion power station, seawater desalination area, offshore floating photosynthetic bioreactor, floating photovoltaic power station, offshore greenhouse, variable pressure diving area, variable floating platform, cultural tourism and entertainment space, underwater evacuation
Key Technologies	Dust suppression technology, structure adjustment technology, 3D printing technology, artificial intelligence-assisted construction technology, building-integrated photovoltaics technology, circadian rhythm lighting technology	Composite skin technology, prefabricated structure deployment technology, lunar soil utilization technology, internal environment control technology, intelligent unmanned construction technology, regenerative life support technology	Marine renewable energy technology, seawater desalination technology, integrated multi-trophic aquaculture technology, anti-corrosion technology, indoor environment control technology, simulation technology
References	[4,27,89]	[25,90,91]	[8,92]

Table 3. Comparison of habitability designs for habitats in extreme environments. The listed extreme environments include Mars, the Moon, and the sea surface.

Design Dimensions	Mars Habitats	Moon Habitats	Maritime Habitats
Extreme Environments	Low gravity, extreme temperatures, strong radiation, dust storms, micrometeorites, Marsquakes	Microgravity, extreme temperature differences, strong radiation, low atmospheric density, lunar dust, Moonquakes	Tidal currents, seawater corrosion, hurricanes/typhoons, ocean current impact, dynamic fluid medium
Design Response	Variable modular units, glass curtain wall with virtual reality interface, building materials produced from in situ resources, power generation by nuclear fission reactors, temperature and humidity control, emergency shelter bunkers, habitat flow field control	Solar photovoltaic panels and thermoelectric power generation, building materials produced from in situ resources, detection and extraction of ice-water on the lunar surface, construction of permanent underground living areas in lunar lava tubes (with connected modular units), digital management	Semi-submersible structures, renewable energy utilization, energy generation and bioproduct production from marine algae, temperature and humidity control, modular design, combined constant–variable pressure structures, oxygen storage, stabilization and mooring
System Composition	Closed life support, energy supply under dusty conditions, air pressurization and supply, waste management, pollution monitoring system	Energy, communication, material supply, waste management, product return to Earth, system operation, self-circulating ecosystem	Energy, water-based food, air, closed-loop water and marine waste management, offshore ecology, circular biological operation system
Functional Modules	Greenhouse ecological park, mobile medical unit, isolation and shelter area, three-level power generation field of nuclear energy–solar energy–wind turbines, power storage device, parking lot, entertainment and social venue, work and laboratory, robot work area	Greenhouse planting base, small animal breeding base, domestic waste recycling facility, oxygen production laboratory, mineral extraction and storage area, biopharmaceutical laboratory, earth-moon relay station, public facilities for medical treatment, education, leisure, commerce and sports	Marine aquaculture base, ocean thermal energy conversion power station, seawater desalination area, offshore floating photosynthetic bioreactor, floating photovoltaic power station, offshore greenhouse, variable pressure diving area, variable floating platform, cultural tourism and entertainment space, underwater evacuation
Key Technologies	Dust suppression technology, structure adjustment technology, 3D printing technology, artificial intelligence-assisted construction technology, building-integrated photovoltaics technology, circadian rhythm lighting technology	Composite skin technology, prefabricated structure deployment technology, lunar soil utilization technology, internal environment control technology, intelligent unmanned construction technology, regenerative life support technology	Marine renewable energy technology, seawater desalination technology, integrated multi-trophic aquaculture technology, anti-corrosion technology, indoor environment control technology, simulation technology
References	[4,27,89]	[25,90,91]	[8,92]

4.2.2. Design of Capsule Space in Extreme Environments

Module buildings feature high strength and self-sufficiency and are more suitable than habitats for small populations to survive in harsher conditions [93]. Given their narrow spaces, long-term stays in extreme environments easily bring major physical and psychological challenges to users—fresh food helps alleviate such issues and thus requires special attention. High operation and maintenance costs restrict spatial dimensions, posing great challenges to individual habitability (e.g., convenience and experience). Therefore, extreme environment module design needs to accurately quantify human body dimensions and functional space requirements, define indicators like extreme spatial size and per capita space volume, and fulfill essential functions while improving functional quality within limited space. Spatial efficiency optimization strategies mainly include screening core functional elements, optimizing functional unit zoning, and integrating/expanding habitable units [94]. Representative cases of habitability design for module spaces in extreme environments are shown in

Table 4. Comparison of habitability designs for capsule spaces in extreme environments. The listed extreme environments include outer space, deserts, and mines with mining disasters.

Design Dimensions	Space Station	Desert Survival Capsule	(Mine-Type) Emergency Shelter Capsule
Extreme Environments	Vacuum environment, microgravity, radiation exposure, far from Earth in low Earth orbit	Water scarcity, strong solar radiation, large day–night temperature difference, few organic organisms	High concentrations of CO and CO2, flammable and explosive gases, shock waves, instantaneous high temperature
Design Response	Closed ecosystem, interface of life support system and resource allocation, atmosphere regeneration, water recovery and purification management, environmental monitoring, support for extravehicular activity, full-life cycle design, multifunctional space, streamline simplification, emergency evacuation plan	Plant transpiration water recovery, biological wastewater treatment, utilization of photosynthetically active radiation, algae/plant waste treatment and food supply, cultivation under high CO2 concentration, light concentrator improving photosynthetic photon flux, circular production of photobioreactor	Impact-resistant steel cavity, double-layer sealed cabin door, escape window, multiple oxygen supply system with high-pressure oxygen cylinder (main)–compressed air (backup)–oxygen candle (emergency), air purification device, acid gas adsorption, explosion protection of external air conditioning compressor
Habitat Creation	Gas environment for respiration and energy exchange, water supply, food supply, sanitation support, microbial environment control, personal protective equipment, fire prevention and control, support for physiological health equipment	Agriculture and aquaculture, fermentation biological reaction device, production and conversion of nutrients and biofuels from biomass, day-night cycle energy supply and storage, gas composition control, thermal balance system, power control	Oxygen supply, air conditioning and purification, temperature and humidity adjustment, fixed/portable environmental monitoring system, power supply from mine power source or uninterruptible power supply, auxiliary facilities (chemical toilets, lighting devices, etc.)
Key Parameters	Energy consumption rate for activities of different intensities, mass of main human intake and release products, heat exchange between organisms and environment	Photosynthetic photon flux efficiency, harvest index, carbon cycle period, utilization rate of closed water cycle, light energy utilization rate of crops	Maintenance time, O2 concentration, CO2 concentration, CO concentration, temperature, daily water consumption per person, daily caloric intake per person
Key Technologies	Automation and robot technology, environmental control technology, biological regeneration technology, etc.	Non-photosynthetic wavelength photovoltaic conversion technology, light utilization technology, intelligent monitoring technology	Oxygen supply source guarantee technology, energy adaptation technology, multi-channel intelligent monitoring technology
References	[81,95,96,97]	[11]	[14]

.

4.3. Habitability Design Methods for Deep Underground Space

4.3.1. Functional Space Design

Section 4.2 has reviewed the habitability design concepts for extreme environments such as space stations and desert survival modules. The technical frameworks formed in aspects like closed-loop life support systems and hierarchical resilience–flexibility guarantees provide common logical references for the habitability design of DUS. However, as stated in Section 4.1, DUS features unique characteristics—particularly in terms of spatial functional configurations, where significant differences exist between 100 m scale and 1000 m scale spaces.

Table 5. Design methods for DU functional spaces at the 100 m and 1000 m scales.

Category	100 m Scale	1000 m Scale	References
Internal traffic corridors	Focus on priority to streamline efficiency and adaptation to spatial scale. Optimize the alignment and width of corridors to ensure the smooth flow of people and logistics, avoiding reduced usability due to narrow spaces or cross-flow.	Emphasize unmanned and intelligent operation. Adopt unmanned rail systems and intelligent obstacle avoidance technologies to minimize manual intervention while real-time-optimizing traffic routes through intelligent monitoring systems to enhance transportation efficiency and safety.	[42]
Static traffic spaces	Highlight evacuation correlation and functional connection. Connect static traffic spaces (e.g., parking lots) directly to emergency evacuation routes and core functional areas (e.g., commercial zones, transportation hubs) to ensure rapid conversion into evacuation assembly areas in emergencies.	No need for static traffic spaces due to focus on strategic facilities; thus, no special design is required.	[98]
Military defense spaces	Strengthen protective reinforcement and resource guarantee. Enhance structural impact resistance through steel supports and explosion-proof coatings while supporting independent energy and material reserve systems to meet safety redundancy requirements for military defense.	Focus on strategic function adaptation. Prioritize the layout of special facilities such as energy storage and nuclear engineering. Spatial design must meet operational requirements under extreme environments (e.g., high temperature, high pressure) while strengthening geological stability assessment.	[99]
Energy conversion and utilization	Follow the principles of integration, stratification, and sustainability. Vertically integrate energy conversion facilities (e.g., ground-source heat pumps, waste heat recovery devices) with other functional layers to achieve cascaded energy utilization and reduce resource waste.	Focus on construction of resource–energy circulation zones. Design closed-loop energy systems (e.g., geothermal energy storage–power generation integration) by leveraging deep resources such as geothermal energy and deep minerals to improve energy self-sufficiency and circulation efficiency.	[85]
Material and energy storage	Make good use of special spatial characteristics. Utilize the advantages of rock mass (e.g., good sealing, stable environment) to lay out material storage warehouses in conveniently accessible areas, adopting a modular design for future expansion.	Target strategic storage. Select areas with high rock mass integrity and low geological risks to design large-volume storage chambers for long-term storage of strategic energy (e.g., oil, natural gas) and emergency materials.	[40,72]
Connecting interface spaces	Address corridor scale and geological constraints. Determine interface section dimensions based on rock mass stability, and use flexible sealing materials to treat gaps to prevent gas leakage or water seepage.	On the basis of meeting corridor scale and geological constraints, additionally strengthen safety guarantees by adding pressure monitoring sensors and explosion-proof valves to cope with high-pressure and high-risk environments in deep layers.	[29]
Agricultural production facilities	Due to limited basic environment (e.g., lack of light, limited space), focus on scientific and educational functions. Lay out small-scale hydroponic vegetable workshops and microbial culture modules to balance popular science and emergency food supply.	Focus on resource extraction and production. Utilize deep minerals and geothermal energy to design mineral extraction and processing workshops, supporting energy recovery systems to reduce production energy consumption.	[100]
Ecological landscape facilities	Focus on construction of deep underground ecosystems. Cultivate shade-tolerant plants through artificial lighting and water circulation systems to create small-scale ecological landscapes and improve spatial psychological experience.	No need for ecological landscape facilities due to its high closure and single functionality; thus, no special design is required.	[101]
Data processing spaces	Require supporting energy and environmental control systems. Provide a stable power supply (with backup power) and a constant temperature environment (temperature controlled at 20~25 °C) for data-processing equipment to ensure normal operation.	No need for telecommunication/data processing spaces due to functional positioning and technical constraints; thus, no special design is required.	-
Other functional spaces	Need to conduct special design for specific scenarios. For example, commercial spaces should optimize pedestrian flow organization, and medical spaces should be equipped with clean systems to ensure functional adaptation.	Following the principle of special design for specific scenarios, as in the 100 m scale, further restrict spatial function types due to more complex deep environments, prioritizing the guarantee of strategic and core functions.	-

, below, summarizes the functional space design methods for deep underground habitability and proposes differentiated design approaches based on the characteristics of these two types of spaces (100 m scale and 1000 m scale). The design methods for different specific aspects will be elaborated in detail in the following text.

4.3.2. In Situ Environment Adaptation Design

This design focuses on passive defense and active regulation and formulates differentiated design strategies for the environments of 100 m scale and 1000 m scale DUS:

• For a 100 m scale DUS, there are no extreme temperature or humidity issues, and the air quality pressure is moderate. However, it is affected by noise sources in shallow-to-middle layers and irregular, unfavorable geological structures. The design requires the following: weakening external noise interference through structural sound insulation measures; conducting the targeted reinforcement of areas with unfavorable geology based on geological survey results [60,102]; and achieving good environmental coordination with shallow-to-middle underground spaces [46].
• In a 1000 m scale DUS, due to increased depth, it exhibits obvious extreme temperature and humidity, complex air composition, and air pressure exceeding the comfort standard. It also faces risks of unfavorable geological structures. The design requires the following: focusing on building active temperature and humidity control systems and air pressure balance mechanisms [29]; adopting engineering measures to address geological structure risks.

4.3.3. Human Habitat Environment Assurance Design

Centered on meeting basic survival needs, this design addresses key issues including air (oxygen supply and hazardous gas treatment), water, artificial lighting environment, pollution (formaldehyde, TVOC, radon, etc.), waste management, and food shortage. The design requires the following:

• Uniformly configuring systems for air purification, water supply, artificial lighting, pollution prevention and control, and waste treatment while stockpiling food resources [14,57].
• Additional adaptations based on depth scales: For the 100 m scale DUS (−30~−100 m), taking cities in Table 2 as examples, cities such as Nanjing and Hangzhou (with a CTZT of 17.9~19.6 °C and a geothermal gradient of 1.9~3.4 °C/100 m) can leverage their natural constant temperature property to optimize spatial heat exchange efficiency, eliminating the need for additional large-scale temperature control equipment. For the 1000 m scale DUS (below −100 m), quantitative data indicate notably extreme environmental conditions. According to Figure 8, the relative humidity at this depth ranges from 80% to 100%, and the CO₂ concentration at −1000 m increases abruptly by approximately 30%. Therefore, it is necessary to configure a constant temperature and humidity system with a dehumidification capacity of no less than 5 kg/(h·100 m²), and adopt variable-frequency ventilation to control the CO₂ concentration at ≤1000 ppms [82,103,104].

4.3.4. Medical and Sanitary Adaptation Design

This design adopts a closed, modularized approach. DUS is restricted by conditions such as waste treatment, energy supply, and facility environment in terms of personal hygiene facilities, human waste management, sleep and health care spaces, and medical treatment spaces. The design requires the following:

• Integrating the above functions into closed modular units to reduce resource consumption and environmental interference. Pollution prevention and control should focus on controlling formaldehyde concentration at ≤0.1 mg/m³ and TVOC (Total Volatile Organic Compounds) concentrations at ≤0.6 mg/m³. For the potential issue of radon concentration exceeding the standard in a 1000 m scale DUS, additional activated carbon adsorption devices should be installed to ensure the radon concentration is ≤100 Bq/m³ [82].
• Emphasizing the connection between protective isolation spaces, civil air defense (CAD) spaces, and emergency evacuation spaces to ensure protection and evacuation efficiency in emergency scenarios [40,81,105].

4.3.5. Hierarchical Resilience and Flexibility Design

Hierarchical resilience and flexibility design addresses resilience differences between two space scales, focusing on risk prevention, emergency adaptation, and reserved space planning.

• Spaces at the 100 m scale prioritize structural reinforcement and vertical coordination of civil air defense spaces, alongside optimizing hierarchical emergency evacuation systems, passage layouts, and sub-safe zone functions. For peacetime–disaster (war/emergency) dual-purpose spaces, they implement systematic resource integration and flexible space reservation, with reserved development spaces pre-equipped with phased construction interfaces for expansion.
• Spaces at the 1000 m scale have no civil air defense or dual-purpose space requirements. Their emergency evacuation design centers on vertical evacuation structures, and reserved development spaces are dominated by strategic spaces without additional phased construction interfaces [100].

4.3.6. Adaptive Operational Facility Design

Adaptive operational facility design ensures operational facilities match space functions, covering shared requirements and scale-specific adaptations.

• Both scales require closed-loop life support systems for basic living conditions, optimized vertical evacuation structures to enhance emergency efficiency, and robotic operation–maintenance systems for automated daily management [11,14].
• Spaces at the 100 m scale prioritize traffic flow efficiency in internal passages to ensure smooth pedestrian-logistics connection.
• Spaces at the 1000 m scale demand higher-standard elevators for vertical evacuation, robotic systems integrated with holographic safety perception and intelligent automation, and internal passages meeting unmanned, intelligent operation standards for large-scale, high-complexity scenarios [98].

4.3.7. Reconstruction Design of Existing Environments

Focused on efficient resource reuse and precise functional adaptation, reconstruction design of existing environments regenerates existing space value through optimized support systems and resource/facility reuse.

• Both space types require adaptive air and water supply system adjustments based on mine pit geology and existing infrastructure, alongside targeted monitoring/control plans to address mining legacy pollution and secondary pollution risks.
• Spaces at the 100 m scale adopt multi-dimensional resource–energy utilization (integrating mineral development, geothermal use, energy storage, and eco-cultural tourism) and conduct comprehensive benefit assessments (abandonment, reuse, new procurement) for existing equipment/facilities before cost–benefit optimization.
• Spaces at the 1000 m scale focus on secondary resource use (e.g., energy/gas/oil storage, CO₂ sequestration), with most existing equipment/facilities requiring retrofitting or demolition–reconstruction due to obsolescence or mismatch [40,51].

4.3.8. Construction and Operation–Maintenance Design

Centered on efficient construction management and intelligent operation–maintenance, construction and operation–maintenance design balances quality and efficiency via differentiated strategies.

Construction process management:

According to Figure 7, the vertical in situ stress follows the formula σ_v = 0.0245H (where H is the burial depth). For a 1000 m scale DUS, this stress can reach 24.5~49 MPa, thus requiring support reinforcement with new-type reinforced materials. Additionally, when the depth exceeds −100 m, the horizontal seismic acceleration is 0.2 times the surface value, which allows for the simplification of shallow-layer seismic resistance measures while retaining vertical redundancy.

• Spaces at the 100 m scale develop material transportation plans considering geological properties and surface routes (balancing economy and safety) and incorporate geological waste disposal sites during construction for centralized handling. For cities with karst geology, targeted grouting reinforcement shall be carried out on karst areas based on survey data during the construction process (Table A3).
• Spaces at the 1000 m scale require pre-planned upper-layer dedicated transportation to solve deep-space delivery challenges, with construction relying on surface prefabricated component assembly and modular micro-spaces to boost efficiency and safety via industrialized methods [26,91].

Operation–maintenance system development:

• Spaces at the 100 m scale focus on daily unmanned equipment management and maintaining equipment space stability for reliable operation.
• Spaces at the 1000 m scale prioritize dedicated unmanned equipment maintenance spaces, using independent areas and specialized equipment to ensure orderly work for complex systems without disrupting main functions.

4.3.9. Ecological and Cultural Adaptive Design

Furthermore, toward livability, DUS design also needs to balance the synergy of function, culture, and ecology. On the one hand, it should align with the inertia of surface living habits and create a localized environment that is close to natural and urban contexts; on the other hand, based on interdisciplinary research findings, it should achieve the integration of artificial and natural environments, explore a cultural context adapted to life in DUS, and pay attention to avoiding spatial homogeneity—making good use of underground topological structures to create spatial sequences.

4.4. Future Research Directions

It should be noted that this study is still in the frontier exploration stage. In fact, the global utilization of DUS in the contemporary era mainly covers municipal engineering, deep underground resource extraction and storage, deep underground metro tunnels, and deep underground laboratories, with no publicly accessible deep underground habitation projects for urban residents having been developed so far. Therefore, verifying design methods through engineering practices is not feasible at present. However, simulation experiments can make up for this practical limitation to a certain extent and provide support for verification. In the future, habitability simulations for DUS can also be carried out in two dimensions: habitats (macro-scale spaces) and capsules (micro-scale spaces).

Macro-scale simulation research mainly relies on computers. By systematically sorting out the interactive relationships between various habitable subsystems, practical sites with typical DUS development needs are selected for targeted modeling. Priority is given to cities such as Shanghai and Tokyo, where shallow underground space tends toward saturation, and geological data are comprehensive. Based on the multi-dimensional indicators of these cities (including DUS eco-geology and socio-economics), multi-agent system software such as AnyLogic is used to build macro-layout models, with special emphasis on their vertical hierarchical relationships. The models need to incorporate dynamic feedback mechanisms—for example, simulating the optimization of personnel evacuation routes and resource allocation efficiency of each module under scenarios such as earthquakes or internal fires—and output the final planning scheme.

Micro-scale research should focus on the extreme environmental characteristics of a 1000 m scale DUS. Given that computer simulations cannot accurately reproduce the complex physical fields in deep underground environments, research approaches such as aboveground simulation cabin construction or collaboration with deep underground laboratories should be prioritized. Aboveground simulation cabins need to accurately replicate the extreme environment of 1000 m scale DUS: the cabin scale should be designed around the habitation needs of individuals or small groups; environmental control systems should be integrated to adapt to high-temperature, high-pressure, and high-humidity conditions; and life support modules (e.g., air purification, emergency oxygen supply) should be embedded. Initial experiments will focus on testing human physiological and psychological adaptation in environments with no natural light and confined spaces (e.g., circadian rhythm maintenance, emotional state monitoring).

In addition, to achieve the higher-level coupling of the three elements (function, culture, and ecology), further research is needed on the dynamic adaptation laws between the in situ environment of DUS at different depths and functional requirements, as well as on quantifying the ecological benefits of functional utilization of in situ resources. Meanwhile, in targeting the characteristics of DUS, it is necessary to explore the coordinated optimization paths for the ecological, functional, and cultural design of deep underground space from perspectives such as ecological coupling, architectural resilience, cultural resilience, Heritage economy, and cultural empowerment, further improving the DUS livability design evaluation system.

5. Conclusions

This study focuses on the challenge of habitability design for deep underground space (DUS)—a typical extreme environment. With the goals of addressing disciplinary gaps, integrating interdisciplinary experiences, and developing practical design methods, it systematically completes the theoretical organization and method construction of DUS human habitat design through bibliometric analysis, multi-environment comparative research, and geological data verification. The main conclusions are as follows:

• Clarifying the characteristics and definition criteria of DUS: Based on Japan’s MLIT and China’s urban planning practices, this study defines the depth of DUS as 30 m below the ground surface. It also optimizes the classification logic by integrating topographic slope, solving the definition problem of underground space development in mountainous areas. Through a survey of geological data from 28 major cities in China, the core environmental parameters of DUS are quantified, and its derived characteristics (e.g., closed isolation and irreversible development) are clarified—laying a foundation for the subsequent proposal of design methods.
• Extracting interdisciplinary reference experiences for extreme environment habitability design: By comparing design practices in extreme environments such as space stations, lunar habitats, desert survival modules, and mine emergency shelters, this study identifies three core common strategies: in situ resource utilization, modular integration, and closed-loop life support. These experiences are adapted to DUS scenarios, and targeted strategies (e.g., geological material reuse, miniaturized closed modules for 1000 m scale spaces, and hierarchical protection systems) are proposed, representing an initial exploration of solutions to the contradiction between resource constraints and safety requirements in deep underground environments.
• Constructing an element system for DUS habitability design: Based on interdisciplinary research findings, this study summarizes five categories of core design elements: natural condition factors, construction status factors, socio-economic factors, user factors, and existing resource factors. This system clarifies the targeted objectives of design and provides a logical framework for the subsequent implementation of methods.
• Developing a demand-oriented DUS habitability design method: To address the differentiated needs of 100 m scale and 1000 m scale spaces, dimension-specific design methods are constructed—covering in situ environment adaptation, human habitat assurance, medical and sanitary adaptation, resilience–flexibility, and existing environment renovation. These methods achieve coordinated adaptation among the deep underground environment, functional spaces, and human needs.

From a practical perspective, this study can provide a basis for functional stratification adaptation in the future development of urban DUS. In engineering design, it is necessary to take geological data as the foundation and draw on the modular design experience of extreme environments to improve the implementation efficiency of habitable systems; meanwhile, a multi-disciplinary collaboration mechanism covering geology, architecture, medicine, and policy should be established to overcome the limitations of design dominated by a single discipline. From an experiential perspective, the habitability design experience of extreme environments needs to be adjusted in combination with DUS scenarios based on the habitability design experience of other extreme environments—after retaining the core logic, it should be adapted to the DUS environment. In view of the “three absences” characteristic of DUS, there is no need to replicate the surface environment; instead, human habitation needs and engineering feasibility can be balanced by simulating natural light, creating small-scale ecological landscapes, and other measures. Notably, this study’s DUS habitability design methodology also contributes to key United Nations Sustainable Development Goals (SDGs): it expands urban space to support SDG 11 (Sustainable Cities and Communities), leverages DUS’s stable environment to reduce carbon emissions for SDG 13 (Climate Action), and promotes in situ geothermal utilization to advance SDG 7 (Affordable and Clean Energy).