Owned by NOAA and managed by the University of North Carolina at Wilmington, the AQUARIUS habitat is an 82-ton double-lock pressure vessel approximately 14 meters long by 4 meters in diameter.

Underwater Outposts
Tech Level: 9
Underwater Hotels
Tech Level: 10
Underwater Base
Tech Level: 12
Underwater City
Tech Level: 14
Underwater Colony
Tech Level: 15
Extreme Depth Habitats
Tech Level: 17

The human race only occupies about 30% of Earth’s surface, namely its land masses. While it makes use of the remaining oceans and seas for transport and fishing, their potential as living space for human beings has only been hesitantly explored.

Underwater habitats have been a viable technology since the 1960s. They have helped to facilitate scientific and technological research, functioned as training grounds for submariners and divers, and helped open up the world under the waves for the common man to see.

The first such habitats were constructed in the early 1960s by Jacques Cousteau and his research team with the backing of the French Petrochemical Industry. Other facilities followed, built by various countries and interests, spurred on by scientific concerns and numerous Cold war projects. When that conflict ended, many were shut down in the wake of decreased need and ever-shrinking budgets. Today, only a handful of underwater habits stay in operation, including the NOAA’s Aquarius Reef Base in the Florida Keys national Marine Sanctuary.

In the near future this may change. Underwater habitats are being seriously considered as major tourist attractions, with at least two underwater hotels under construction. If these facilities become a success, more would likely follow. As environmental concerns with the oceans increase, new facilities may be set up to study the intricate underwater ecosystems in numerous locales. Ocean-based farming is also becoming increasingly popular, and permanent or semi-permanent sub-surface facilities may be constructed to allow full-time tending by operators.

There has also been talk through the years about true underwater communities, envisioned by corporations as showcases, by scientists as learning centers, by military men as covert bases, by isolationist groups as refuges, and by adventurers as a new frontier. A number of political, economic, and technological barriers have kept that from happening as yet, but that may change as techniques advance in the coming decades.

Underwater bases are beloved motifs of science fiction. The Abyss, Sphere, Deep Blue Sea, and a host of other movies have taken place in underwater facilities. Sub-oceanic habitats are also seen in TV series such as Seaquest DSV, Ocean Girl, and the cartoon Sealab 2020 (and its tongue-in-cheek Adult Swim parody Sealab 2021.) The novel Oceanspace by Allen Steele also takes place in an advanced open-pressure underwater research base. The novel Saturn’s Race by Larry Niven and Steven Barnes also features a large underwater habitat as part of an artificial island.


Subaquatic environments are just as complicated and just as potentially hazardous to human life as deep space. In other words, they are one of the most hazardous environments that humankind currently has access to. And unlike space, homesteaders on the subaquatic frontier will also have to deal with a host ecological issues as well.

Like space stations, underwater bases have to deal with a number of factors in order to make humans comfortable enough to live and work productively.

Types: There are three basic types of underwater habitats.

The first, open pressure habitats, exactly counter the pressure of the surrounding water throughout their volume, usually by means of an easily-accessible moon pool, though some also take advantage of airlocks as well. In these habitats, the air pressure is equal to the outside water pressure, and special gas mixtures may be necessary for facilities located below a certain depth. The main advantage of this set-up is that it allows divers easy access in and out of habitat without decompression procedures. However, access to the surface requires decompression. An example would be the US’s old SEALAB facility, as well as the underwater mobile mining station in the movie The Abyss and the sub-oceanic base in the novel Oceanspace.

Most open-pressure habitats operate near the surface, usually within 30 meters of it, where pressure acclimation is usually not a major issue. However, some will occasionally be located deeper. Open pressure bases are usually cheaper and easier to construct and maintain than closed pressure ones.

Closed pressure habitats usually maintain an internal air pressure similar to that of the surface, and access in and out of facility is controlled through airlocks. The main advantage of this set-up is that it allows easy access between it, like-pressured submersibles, and the surface without having to go through pressure acclimation. The main disadvantage of this set up is that it require much more extensive and robust pressure hulls and life-support systems, and any dive would require decompression acclimation. Underwater hotels such as those proposed for Fiji and Dubai, which are designed with casual tourists in mind, would be closed-pressure habitats.

A third type combines both closed and open pressure schemes, with one part of the facility, usually that associated with diving operations, built around and open pressure scheme, while the rest is enclosed in a closed-pressure design. Airlocks with adjacent decompression chambers would separate the two. Such a facility would be more expensive than the other kinds, having to incorporate two types of life support systems, but would offer the advantages of both kinds of habitats in one facility. For example, the closed-pressure section would greatly facilitate the sending and receiving of supplies and personnel from submersibles from the surface, while open-pressure work areas can tend to the needs of divers as they work outside the habitat. Most large underwater habitats depicted in science fiction are combined-system habitats.

Pressure Hulls: Though underwater habitats are often compared to space stations, there is one major critical design difference: space stations need to keep the atmospheric pressure within them from getting out, whereas aquatic habitats need to keep the outside water pressure from getting in. Though on the surface these seem to be similar concerns, they represent dramatically different design philosophies, especially when dealing with extreme conditions.

Pressure hulls underwater need to have rigid, reinforced structural skeletons and need to be able to brace themselves against any possible buckling pressures from outside. They are usually designed to disperse the structural stress evenly over their surface area, hence many underwater vehicle and habitats designed to be rounded and symmetrical, like tubes, ovoids, and spheres.

Currently pressure hulls used for underwater habitats are reinforced steel, but more advanced versions may use lighter and tougher composite alloys or plastics, and graphene or carbon nanotubes may also end up being used in the decades to come.

Air: Most underwater habitats have been supplied air mostly through bottled tanks or by umbilicals to the surface. Advanced facilities in the future may take advantage of artificial gill technology in order to draw breathable air right from the surrounding water. Most modern facilities use atmospheric recycling technology, similar to that on rebreather rigs, allowing them to keep the occupants in breathable air for nearly as long as the habitat’s power and filters can hold out.

Gas Mixtures: The normal sea-level atmospheric gas mixture of nitrogen-oxygen becomes hazardous beyond a certain depth, and could lead to potentially devastating consequences such as the bends. While closed-pressure habitats don’t generally have to worry about this, open-pressure facilities need to take every nuance of pressure effects into account in order to keep their human occupants healthy and active.

Open pressure habitats use many of the techniques pioneered for saturation diving, but geared up for a large, multi-person facility. Much more detailed articles on saturation diving and pressure effects on divers can be found in the links at the end of this article.

Most commonly, helium-oxygen or hydrogen-helium-oxygen mixtures are used at extreme diving depths (usually 50 meters or more below the surface), the ratio depending on the exact pressure the diver experiences and how long he stays at depth. Once acclimated to the pressure at a certain depth, a diver shows no ill physical effects, though long-term health hazards may come into play if he is there for too long, such as aseptic bone necrosis. Also, the helium-oxygen mix does have some inherent disadvantages, such as heat retention issues and distorting voices.

The deepest an open-pressure habitat has operated has been at 183 meters. Experimental tests show that dives down to 600 meters or beyond may be possible, but the divers in those experiments experienced trembling and memory loss from their experience. Its safe to assume that beyond 200 meters or so of depth, closed pressure facilities are necessary for the purposes of human habitation.

Power: Most subaquatic habitats have so far used either batteries, or umbilicals from the surface, or both, to provide energy needs. A number of such facilities from science fiction have also used nuclear reactors. In the future, more advanced bases may use tidal turbines, OTEC generators, or surface wave generators for electricity, or may rely on geothermal taps in certain advantageous locations.

Temperature: Maintaining a comfortable work environment would be essential for inhabitants. Water is forty times denser than air, and tends to easily drain away heat from any source, especially in the deeper parts of the ocean. Besides just being pressure-resistant, hulls of underwater habitats would also have to be heavily insulated to help keep the interior comfortable for its inhabitants.

Construction: All underwater facilities have to date been manufactured in whole on the surface and then lowered into place with cranes and cables. Very large facilities in the near future may be constructed similarly to the ISS, with many smaller modules interconnecting to form a larger base. No one has yet tried manufacturing or assembling a facility from raw parts completely underwater, though for some of the larger habitats discussed later that may become a necessity.

Access: Some habitats can only be reached by scuba diving. Some bases can be accessed by using a vehicle that docks with one or more of the habitat’s airlocks. Some closed pressure facilities may utilize elevators or submerged walkways that go right up to the surface, especially if near shore.

Ecological Considerations: Undersea habitats have to worry about something that their conceptual cousins, space stations, do not: the ecological health of their surrounding environment.

Ocean ecologies are already under threat from a variety of sources; over-fishing, pollution, invasive species, and global warming among them. Designers and operators of any underwater facility have to proceed from a much more ‘green’ perspective now than their Cold War predecessors. Most agree that healthy ecosystems and biodiversity represents the true worth of the oceans, and trying to minimize wide spread destruction of such should always be one of the top considerations with the design of an underwater facility.

Waste management is a big issue, especially with larger bases and colonies. Smaller habitats can just store their waste and send it to the surface for disposal. True underwater cities will have to find other ways of dealing with this, either through stringent recycling and/or using organic waste to help fuel aquaculture (ocean-going farming) projects.

But even so, just the presence of a lot of humans and their machines is bound to have an impact on surrounding marine life, and the more people who take up residence there, the bigger the impact will. This could take many forms. For example, humans may scare off many of the big predators, allowing certain kinds of harmful creature like jellyfish to flourish that were previously kept in check. Waste heat from the habitat and human activity may encourage the runaway growth of certain types of microorganisms in the water, making it toxic to certain species of fish. Industrial accidents could cause devastation to the surrounding ecology, as in the case of oil spills or mid-ocean plastic garbage accumulations.

Any underwater human habitat, especially the larger kind, would have to weigh potential benefits against the potential damage these conditions could cause. After all, besides there is more than just moral and bigger environmental concerns at stake; damaging the surrounding ecology could also jeopardize many habitats’ economic viability.

Tech Level: 9

Outposts are designed for a handful of human inhabitants, usually not more than a half dozen or so, and usually for short-term stays. Durations for crews are typically only a wekk or less, though some have experimented with crews staying for up of to two months. Almost all underwater habitats constructed in the real world so far fall into this category. Almost all were open-pressure habitats.

The purposes of an outpost can be many and varied. These may include:

-- Development of advanced diving techniques and subaquatic habitat technology

-- Research into underwater construction and salvage

-- Military research

-- Studying sea life and ecology

-- Dolphin training and cetacean research (Sealab used trained dolphins for various tasks)

-- Construction, maintenance, and repair of off-shore drilling platforms

-- Construction, maintenance, and repair of underwater industrial or military assets

-- Espionage

-- Laying, and maintenance of, underwater cables

-- Climatological research

-- Tourism

-- Subsurface and seabed aquaculture

-- Submersible maintenance and resupply

-- Training facilities for astronauts

Most outposts are constructed whole on the surface and lowered into place once finished, with divers and automated submersibles usually fine tuning its final placement and anchoring. The seabeds where they’re placed are often scouted and prepped before hand to accommodate the new structure with minimal possible complications.

Most outposts are also dependent on surface umbilicals for power, communication, and sometimes for air cycling and replenishment, depending on the exact systems used. This helps to reduce cost for both construction and maintenance, but does lessen the outpost’s ability to handle unexpected or emergency situations.

Tech Level: 10
The proposed Poseidon underwater hotel under construction off of Fiji.

A number of projects are underway to construct underwater hotels. A more extensive article detailing several of these are linked to at the bottom of this page.

Large underwater hotels are envisioned to be mostly closed pressure structures, with easy access to the surface, usually with elevators going directly between the surface and the seafloor portion of the hotel. They will likely be constructed within easy reach of the shore, and will probably be not placed so deep that pressure issues would greatly increase manufacturing and engineering costs.

Clients would be able to rent various rooms, often with spectacular panaorama views opening onto the surrounding environment. Many hotels would artificially cultivate the surrounding lifeforms to give guests as spectacular a variety of sea life to view as possible.

Tech Level: 12

A base is a larger facility than an outpost, designed for continual occupation by crews which may stay for many weeks or months at a time. To extend the space station analogy a bit, an underwater outpost would be similar to single-launch stations from early in the space age, like Salyut or Skylab. An underwater base would be the equivalent of the multiple-module Mir or ISS.

Various enterprises may require more than just a handful of personnel on site underwater. Such projects may include larger habitat construction, military bases, large oceanographic laboratories, underwater mining facilities, and extensive subsurface and seabed aquaculture. Bases may use open or closed pressure systems, but it seems a combined systems scheme would be most advantageous, so the crew could live and work in comfort when not out on a dive.

As bases would usually be meant to hold more personnel for longer, they would also be constructed considerably larger. In another analogy to current space stations, they would likely be construction in modular form on the surface, which would then be lowered into the water and assemble together with divers or Underwater Autonomous Vehicles (UAVs.) The exact design and number of modules would depend on the base’s purpose, but individual components would likely be no larger than shipboard cranes and transport facilities could handle.

Bases could also be constructed on-site from raw building components, but would take considerably longer and would require many more man-hours in the water to accomplish. Using UAVs for the same purpose would likely take even longer, at least with current or near-future technology. The modular approach is likely in the end to be much cheaper until underwater working and construction techniques improve significantly.

Like with outposts, the base would still have extensive umbilical connections to the surface for power, air, communications, and so on. However, because they have to provide for many more people, bases may be designed to be more self-sufficient in some areas, such as power or air production through artificial gills, in order to help mitigate problems should the umbilicals fail.

Tech Level: 14

An underwater city leaves behind the idea of pure-utilitarian notion of underwater habitats, and is a full-blown effort at long-term residency under the waves. The definition of a ‘city’ is used pretty loosely here as well, meaning any fairly large undersea community, from a few hundred to a few thousand inhabitants. It may be a single unified construct, like the classic domed underwater cities of science fantasy, or they may just be centralized loose conglomerations of bases, outposts, and other structures.

Though today there seems to be little call for large scale oceanic habitation beyond a few isolated dreamers, in the future needs and attitudes may change, and advancing technology may make such projects much cheaper.

For example, artificial island projects may proliferate, either as adjuncts to deep ocean oil drilling, OTEC generator platforms, tourist destinations, military bases, or the like. Sub-oceanic communities may spring up in or around these, as large underwater structures would already be in place, and may be built to accommodate human inhabitants as part of their design.

Underwater cities may also become a natural outgrowth of seabed aquaculture, as subsurface farms and fisheries grow ever larger in size.

Underwater cities may also be built with military advantages in mind. Hidden under hundreds of meters of water, a military base or population center may also be much harder to find or attack.

Coastal cities may also end up expanding into the water of their harbors just from various population and economic pressures. At first it may start as just tourist attractions, expansions of docks, and residencies for the rich, but as the technology advanced and the costs come down, they may build more affordable sub-aquan neighborhoods if expansion in other areas may be blocked.

If the worst predictions of global warming come true, many coastal communities and cities may end up permanently flooded or even completely underwater. Subsurface communities may spring up in and around these ‘zombie’ cities, intent on salvage and recovery, perhaps recycling old buildings and materials for their underwater habitats.

Underwater cities will have to be far more self-sufficient than their smaller cousins. Umbilicals to the surface could supply enough power to a large community, but consumables such as air and drinking water may be another matter entirely. And even so, the city having its own dedicated power sources would be a necessity in case of any kind of emergency.

Underwater cities would most likely be almost entirely closed pressure affairs, with only divers’ work areas having an open-pressure scheme. The large dome cities of golden age scifi would seem to be impractical as the domes would seem to be too vulnerable to wear and damage over time. Instead, for large open spaces a subaquan city may dig into the seabed instead and have a number of levels beyond just the visible constructs on the ocean floor.

Tech Level: 15

Undersea colonies are collections of various underwater habitats of various sizes, which may or may not be physically interconnected, whose population generally runs into the thousands. The difference between a city and a colony are two fold: suboceanic colonies are completely independent of the surface, and are designed to expand on their own.

By being independent, this is not to say that the colony would have no contact with the surface. Indeed, the colony would still likely trade and receive visitors and tourists and the like from surface interests. But it will be able to tend to all its vital needs and functions itself. Food will be cultivated through various aquaculture techniques and drinking water would be desalinated on-site. The colony would likely have several different local power sources available, and be able to extract breathable air from the surrounding water or seabed. It would also have a number of mining and manufacturing industries to fabricate the parts and machines it would need to keep itself going. Recycling would be as efficient as possible.

In other words, it would be a miniature world unto itself, in some ways not unlike the grand space colonies visualized by Gerard O’Neill and others.

The majority of the colony would likely be closed-pressure systems, very similar to surface conditions in order to bring up families with a minimal of medical complications.

At Tech Level 15, when truly independent undersea colonies become possible, pressure hull and aquatic life support technology should allow for long-term establishment of large closed pressure habitats down to 250 meters or so. This would open up much of the continental shelves to potential human occupation, and entirely new territories and nations may arise under the oceans.

Tech Level: 17

Eventually, materials and habitat technology may advance to the point that outposts could be set up just about anywhere in the ocean, even at its lowest and most crushing depths. Chances are these wouldn’t be very large affairs, as at this Tech Level, automatons would be able to do just about anything a human could do, if not better. Humans may venture into these depths only to supervise and coordinate these super-advanced UAVs. Purposes for such extreme depth habitats could include mining, exploration, and scientific research.










Underwater Hotels


Artificial Gills


Article added 16 May 2010