A subterranean city is any large-scale built environment designed for sustained human activity beneath the earth’s surface, ranging from ancient hand-carved refuges to the climate-controlled pedestrian networks beneath modern metropolises. The idea sounds like science fiction, but people have been living, working, worshipping, and commuting underground for thousands of years. What has changed is not the impulse to go below ground but the scale and sophistication of the engineering, the understanding of what prolonged underground living does to the human body and mind, and the growing economic and environmental arguments for pushing cities downward rather than outward.
Why People Have Always Gone Underground
The oldest known subterranean cities were carved into the soft volcanic rock of Cappadocia, in modern-day Turkey. Derinkuyu, the deepest and most famous, extended multiple levels below the surface and contained living quarters, warehouses, schools, religious spaces, water wells, ventilation shafts, and even stables for animals.1Procedia Engineering. The Study of Derinkuyu Underground City in Cappadocia Located in Pyroclastic Rock Materials The pyroclastic tuff was soft enough to carve with hand tools yet firm enough to stand without collapse for centuries. These were not primitive holes in the ground; they were planned settlements with airflow and sanitation designed into the layout from the beginning.
The motivations that drove the Cappadocians underground still resonate. They sought protection from invasion, shelter from extreme surface temperatures, and the ability to store food in naturally cool conditions. Today’s reasons are updated versions of the same list: protection from traffic congestion, shelter from harsh climates, and the desire to free up surface land for parks and greenery rather than parking garages.
Engineering the Ground Beneath a City
Cutting a small chamber out of cooperative rock is one thing. Excavating a large underground space beneath an existing urban area, where the ground is full of water, utilities, and building foundations, is a different problem entirely. Modern subterranean construction relies on methods that can handle unpredictable geology without destabilizing what sits above.
One widely used approach is sequential excavation, where tunnels are dug in stages with support installed after each small advance. When conditions are especially difficult, engineers turn to more specialized techniques. At the Naghshe Jahan Square subway station in Isfahan, Iran, the presence of a high water table forced engineers to evaluate compressed air and ground freezing to stabilize the surrounding soil, ultimately proposing a large-diameter curved pipe roofing method to keep the excavation from collapsing inward.2Journal of Rock Mechanics and Geotechnical Engineering. Feasibility evaluation for excavation of Naghshe Jahan Square subway station by underground methods In Italy, another approach called ADECO-RS was used to push through highly swelling and squeezing ground for high-speed rail tunnels, including a face area exceeding 140 square meters under 500 meters of cover between Bologna and Florence.3Tunnelling and Underground Space Technology. Sequential excavation, NATM and ADECO: What they have in common and how they differ
Even after excavation, the structure must cope with ongoing geological forces. Research on large underground caverns has shown that dynamic loading, such as from seismic activity, causes failure patterns that depend heavily on the orientation of weak planes in the surrounding rock. Interestingly, having multiple caverns side by side can sometimes improve overall stability rather than weaken it, because the intervening rock pillars redistribute stress.4Journal of Rock Mechanics and Geotechnical Engineering. Dynamic failure modes of large-scale underground caverns with complex geological structures Groundwater is another persistent headache. One design solution integrates the groundwater bypass directly into the bottom slab of an underground structure, replacing a conventional gravel drainage layer with a gridded network of canals that both relieve water pressure and serve as the structural foundation.5Tunnelling and Underground Space Technology. Integration of groundwater by-pass facilities in the bottom slab design for large underground structures
The Natural Climate Advantage
One of the strongest arguments for building underground is thermal stability. Soil temperature barely fluctuates compared to air temperature, and the deeper you go, the more stable it gets. Research in southern Algeria measured this directly: at the surface, summer air temperature reached 46 °C, but just two meters down the soil held steady at about 30 °C year-round. An underground room modeled at that depth stayed between roughly 12 °C in winter and 27.5 °C in summer, a far narrower swing than above ground.6Case Studies in Thermal Engineering. The geothermal potential of underground buildings in hot climates: Case of Southern Algeria That means dramatically lower energy demand for heating and cooling, which in desert climates can account for the majority of a building’s energy consumption.
This thermal buffering is not limited to hot regions. In cold climates, underground spaces stay warmer than surface buildings in winter for the same reason: the earth insulates against temperature extremes in both directions. The Cappadocians understood this intuitively. Modern engineers quantify it with thermal simulations, but the principle has not changed.
The catch is that the underground environment is not naturally comfortable in other ways. Humidity tends to be high, natural light is absent, and air does not circulate on its own. Maintaining livable conditions requires active ventilation, dehumidification, and artificial lighting systems that add cost and complexity.7E3S Web of Conferences. Ventilation and environmental control of underground spaces: a short review The energy saved on climate control can be partially offset by the energy spent on these support systems, though the net balance still tends to favor underground construction in extreme climates.
What Living Underground Does to People
Thermal efficiency is meaningless if people cannot stand being down there. The psychological and physiological effects of prolonged underground habitation are a real constraint, and the research paints a mixed picture.
A study of workers in a deep gold mine in China found that despite reporting physical discomfort from heat, humidity, dim lighting, and confined spaces, their overall mental health scores were not worse than the national average for Chinese men. But the relationship was dose-dependent: the more environmental stressors workers perceived, and the longer they spent underground continuously, the higher their psychological distress scores climbed.8PubMed Central. Subjective perceptions and psychological distress associated with the deep underground: A cross-sectional study in a deep gold mine in China In other words, short stints were tolerable, but extended exposure accumulated into something more concerning.
A separate study tracking people over seven consecutive days in an underground working environment found that emotions deteriorated in a cumulative pattern, with vitality declining steadily and fatigue building up over time. Mood disturbance was closely linked to worsening sleep quality.9Building and Environment. Evolution of physiological, psychological, perceptual, and cognitive performance during a seven-day residence in underground working environment This suggests that the underground environment disrupts circadian rhythms, even when temperature and air quality are controlled.
Hormonal data supports that interpretation. Research comparing underground and surface workers found that the annual cortisol rhythm, which normally tracks seasonal changes in daylight, was much flatter in the underground group. Their melatonin cycle was also altered, with a much larger swing between day and night levels compared to surface workers. And the underground workers slept almost half an hour more per night, possibly a compensatory response to disrupted circadian cues.10Environment International. The subterranean work environment: Impact on well-being and health Researchers studying subterranean design have emphasized that bringing natural daylight into underground spaces is not just an aesthetic preference but a matter of physiological and psychological health.11Advanced Engineering Forum. Conceptualized Technological Solutions to Maximize the Use of Natural Daylight and Provide a Sustainable Environment for Vehicle Transportations in Modern Underground Cities: A Review
None of this means underground spaces are inherently unhealthy. It means design has to actively compensate for the absence of natural cues that surface dwellers take for granted: sunlight, temperature variation, open sightlines, and a sense of spatial freedom. The challenge is engineering those qualities back in, and many modern underground spaces do so effectively.
Modern Underground Pedestrian Networks
The most familiar form of the subterranean city for most people is not a deep excavation but a pedestrian network just below street level. Montreal’s underground city, known locally as RÉSO, is one of the largest, connecting office towers, shopping centers, metro stations, and hotels through an extensive system of indoor walkways. A study of pedestrian movement through the system found that the spatial distribution of foot traffic was remarkably stable from day to day, with only minor differences between weekdays and weekends and no strong directional flows over time.12Journal of Transportation Engineering. Modeling Pedestrian Dynamics in Montreal’s Underground City The system functions less like a transit corridor and more like a self-contained urban environment with its own predictable rhythm.
What makes these networks interesting from a planning perspective is that people do not always prefer them over surface routes, even in harsh weather. Research into multi-level pedestrian environments has found that the factors driving underground route choice are still not fully understood, making it difficult to model how people will actually use these spaces.13Concordia University Spectrum Research Repository. Predicting the level of use of underground routes in a multi-level urban environment Convenience, perceived safety, lighting quality, signage, and even ceiling height all seem to play a role. Montreal’s system works in part because it evolved organically over decades, growing outward from transit hubs and commercial anchors rather than being imposed as a single master plan.
Fire Safety and Evacuation Below Ground
The nightmare scenario for any underground space is fire. Smoke rises, but in an enclosed underground environment it also spreads laterally, filling corridors and reducing visibility to zero. Evacuation routes that seem simple on a map become disorienting when you cannot see.
Simulation studies of underground fires provide some sobering numbers. In a model of fire in the underground space of Guangzhou International Financial City, researchers found that when all firefighting systems failed, the available safe evacuation time for people was about 530 seconds, or just under nine minutes, before conditions became lethal.14Fire. Underground Evacuation and Smoke Flow Simulation in Guangzhou International Financial City during Fire Separate research on an underground metro transfer station in Wuhan modeled two fire scenarios, a flammable package and an equipment short circuit, analyzing smoke movement, temperature, visibility, and carbon monoxide to work backward from critical safety thresholds to determine how much time occupants actually have to get out.15Buildings. Smoke Flow and Evacuation Safety in the Event of Fire in an Underground Rail Transit Transfer Station
These studies consistently show that the margin for error underground is thin. Surface buildings allow smoke to vent upward and give occupants the option of moving laterally or even breaking windows. Underground spaces offer none of those escape valves. Mechanical smoke extraction systems, compartmentalization of spaces, redundant exit routes, and real-time monitoring are not optional extras for subterranean cities. They are baseline requirements. Any serious proposal for underground habitation or commercial use lives or dies on how well it handles fire.
Sustainability and the Carbon Question
One argument gaining traction for underground development is its carbon footprint. A national-level analysis in China found that from a life-cycle perspective, underground space development does not lead to higher carbon emissions compared to equivalent surface construction. Underground transit and commercial facilities reduce pressure on surface development, and the surface area freed up can be converted to green space, adding plant-based carbon sequestration that further offsets emissions.16Engineering. Role of Urban Underground-Space Development in Achieving Carbon Neutrality: A National-Level Analysis in China
Underground waste management adds another sustainability dimension. Automated vacuum waste collection systems use air suction through a closed network of underground pipes to transport waste from drop-off points throughout a city to a central collection station, reducing greenhouse gas emissions from garbage trucks along with the noise and odors of conventional collection.17Environmental Modelling & Software. Modeling energy consumption in automated vacuum waste collection systems Cities like Stockholm and Barcelona have operated versions of these systems for years, routing waste pneumatically so that trucks never need to enter residential neighborhoods.
The sustainability case is strongest when underground development replaces what would otherwise be surface sprawl. If a city can put its parking, logistics, and transit infrastructure underground and preserve the surface for walkable streets and parks, the net environmental result is likely positive even after accounting for the energy cost of excavation and the ongoing power demands of ventilation and lighting.
The Slow Rot Below
Underground structures age differently than surface ones. One of the less glamorous challenges is biological degradation of concrete. Microorganisms, particularly sulfur-oxidizing bacteria found in sewer environments, produce sulfuric acid that eats into concrete over time. This process can reduce the lifespan of underground infrastructure from a designed life of 100 years to as little as 30 to 50 years, depending on how harsh the environment is. The same microbial activity generates hydrogen sulfide, carbon dioxide, ammonia, methane, and other volatile compounds, which pose health risks for maintenance workers.18PubMed Central. Microbiologically Induced Concrete Corrosion: A Concise Review of Assessment Methods, Effects, and Corrosion-Resistant Coating Materials
For a subterranean city, this is a maintenance problem that never goes away. Surface buildings degrade from weather, UV exposure, and freeze-thaw cycles. Underground structures degrade from water intrusion, chemical attack, and biology. Corrosion-resistant coatings and antimicrobial concrete mixes exist, but they add cost and need periodic renewal. Any city that moves significant infrastructure below ground is signing up for a permanent and expensive maintenance obligation that is harder to inspect and more disruptive to repair than anything on the surface.
Who Owns What Beneath the Street
Building underground raises a legal question that surface construction rarely encounters: who owns the space below the ground? In many legal systems, land ownership historically extended from the surface to the center of the earth in theory, but in practice, rights to underground space are poorly defined. In China, where underground development is advancing rapidly, all urban land is registered as state-owned, and land-use rights cannot be separated from the ownership of the buildings or structures above them. This means there is no clear legal framework for independently registering, transferring, or financing underground space as a distinct property right, which creates significant obstacles for private investment in subterranean development.19Elsevier / Land Use Policy. Property rights of urban underground space in China: A public good perspective
Other countries handle this differently. Some jurisdictions have adopted “stratum titles” or volumetric property rights that allow ownership of a defined three-dimensional block of space, whether above or below the surface. Without such frameworks, underground development tends to be limited to government-led projects because private developers cannot secure the legal certainty they need to justify the investment. The legal infrastructure is, in some places, further behind than the engineering.
Subterranean Cities Beyond Earth
Perhaps the most dramatic frontier for underground habitation is not on Earth at all. The Moon and Mars both have lava tubes, naturally formed tunnels left behind by ancient volcanic flows, that could serve as ready-made shelters for human settlements. Lunar lava tubes are of particular interest because they offer natural protection from radiation, micrometeoroid impacts, and the extreme temperature swings of the lunar surface, where daytime heat and nighttime cold span hundreds of degrees.20Acta Astronautica. Pressurized lunar lava tubes for habitation
Simulations have shown that when the rock overhead is between one and six meters thick, the harmful radiation effects of solar particle events and galactic cosmic rays on the human body become negligible, meaning no additional shielding equipment would be needed inside a lava tube of sufficient depth.21International Journal of Mining Science and Technology. A comprehensive review of lunar lava tube base construction and field research on a potential Earth test site Engineering analyses suggest that small lava tubes could even be pressurized with breathable air while maintaining structural integrity, essentially creating a sealed underground habitat from a natural geological feature.22Acta Astronautica. Pressurized lunar lava tubes for habitation The cross-sectional shape of the tube matters: circular or vertically elongated elliptical profiles are the most structurally stable under pressure.23International Journal of Mining Science and Technology. A comprehensive review of lunar lava tube base construction and field research on a potential Earth test site
There is a certain symmetry to the idea. Thousands of years ago, people carved underground cities out of volcanic rock to escape danger on the surface. The first permanent settlements on other worlds may do something remarkably similar, retreating into volcanic tunnels for the same basic reason: the surface is too hostile, and the underground offers a natural head start on the shelter problem that every civilization has to solve.