Rapture, Andrew Ryan’s underwater city somewhere near Iceland and just one of many examples of the humanity utopia of living under the sea. While this kind of project remains a dream, there have been some (less ambitious) approaches to underwater cities, like Amsterdam Underwater Project. The idea here, rather than living underwater, is to drain the canals, use the extra building space and fill them back. Since the construction is not really deep and it is connected to the surface city, this approach neatly avoids all the problems related to real underwater cities, like pressure and decompression (see this post on Abbyss to review the decompression issue), and regenerative systems for air, water, food, electricity, and other resources, and only needs to focus on ventilation and air conditioning. However, recent underwater habitats allow for required resources to be delivered using pipes, or generated within the habitat.
Construction of a whole city underwater is obviously, out of the table, but there have been several projects to create underwater habitats, most of them operating as marine labs. This idea has been constantly repeated in scifi movies and TV series like the Abyss, Deep Blue Sea, Seaquest or, more recently, Lost, not to mention they seem to be the second best location for evil fortresses, after volcanic tropical islands (see, for example, Mazinger Z, The Spy Who Loved Me or Call of Duty Black Ops).
Actually, underwater labs have been there for a while since the 60s. To name a few well known ones, there were Conshelf I-III, SEALAB I, II and III, Hydrolab or Aquarius.
The main concern in these early labs was basically providing breathing air of suitable quality, usually by means of bottled tanks or by umbilicals to the surface. Most modern facilities use atmospheric recycling technology, similar to that on rebreather rigs, to extend the use of available air.
Coping with pressure was a close second too. Underwater lab hulls need to have rigid, reinforced structures to resist the outside pressure. This is usually assisted by a round structure to disperse the structural stress evenly over their surface area. In really deep labs, health is a main concern, since pressure becomes a a real issue. The deepest an open-pressure habitat has operated has been at 183 meters. From this point below, closed pressure facilities -where one can keep a pressure closer to the surface one- are required. This is possible by closing to the sea via hatches and an airlock, so that the sea won’t flood the place, whereas older underwater structures were actually open to ambient pressure via a moon pool, meaning the air pressure inside the habitat equals underwater pressure at the same level. However, a closed pressure habitat also implies way stronger structures to prevent implosions.
Providing heating, power and food and disposing of waste products comes next. In most existing cases, these needs were supplied from the surface, but there are (futuristic) plans to make underwater installations self sufficient.
If you are not a scientist or a supervillain, you can still spend a night under water in a hyper-luxurious 10 star hotels under the sea. Of course, one of the first cities to harbour such a project had to be Dubai.
The Hydropolis Underwater Hotel and Resort was designed by Professor Roland Dieterle to be built 20 m below the surface of Persian Gulf. The approximate cost of the project quickly went from 300 million USD to almost 600 million and, as far as I know, the project has been dropped.
The access was supposed to start in a 120 semicircular cilinder (land station) that would bring visitors to the bottom level, where they could board a silent train pushed by wholly automated cable together with a modular, self-helping steel pathway to Hydropolis.
Another long delayed project, although still not openly discontinued is the Poseidon resort in Fiji, 40 feet underwater, designed by American submarine engineer Bruce Jones. For 1500 USD a night, one can spend a night in a standard suite in the lagoon. The key idea in this case was to keep a one-atmosphere at the resort -the same as in the surface-, so no decompression would be necessary.
Believe it or not, there is actually one operating underwater hotel, but it looks more like a youth hostel than like the previous luxury resorts; after all, it used to be a used to be a marine lab. This one is called The Jules Undersea Lodge and it is located in Key Largo, Florida. In order to get to this one, the visitor needs a crash introduction to scuba diving: the entrance to the hotel is 21 feet underwater on the sea floor. Obviously, the Lodge offers extensive scuba certification courses and diving excursions. And one does not have to worry about deco until it’s time to go back to civilization.
Visit Orbital Vector for more information on underwater habitats.