Underwater

While there are some neat benefits to an undersea structure, there are a large number of disadvantages and engineering challenges as well.

Advantages

• Protected from the elements: No waves or wind. Most living creatures on the ocean live either above the water (birds) or below it (fish). The air/water interface (as engineers call it) is a brutal place, and avoiding it is a huge win.
• Scenic: Will have a great view if it’s in the right location.
• Unique: Underwater resorts draw customers. For example, Jules Undersea Lodge off Key Largo, Florida, charges $300-$600/night.
• Hidden: Can be very isolated, defensible and hard to find, if desired.
• Cheap to expand: By using clever construction methods, expanding the colony volume might be fairly inexpensive. A variation on the technique used to build Monolithic Domes seems promising [MDI]. Fill a plastic form with water, then spray or layer on concrete and insulation. When the concrete has set and can take the pressure, pump the water out and pump air in, and you have a new airspace. If the colony is not very deep (so water pressure is low), the form can simply be inflated with air from the beginning.

Disadvantages

• Bad failure mode: If other seastead designs fail, residents end up on top of the ocean. If an underwater structure fails, the ocean ends up on top of them!
• Psychological difficulties: Access to fresh air and sunlight will be difficult, and combined with lack of open space makes for a very challenging environment for humans. Submarines, for example, are notoriously challenging psychological environments. Some air and sunlight can be piped in from the surface, but getting large amounts of sun will require large structures at the surface.
• Additional engineering difficulties: Building a seastead is already difficult. Building an underwater one will be even harder, especially at any significant depth (more than 200 feet or so).
• Difficult to get in and out: If the structure is kept at atmospheric pressure, it has additional structural stresses. If it is kept at the appropriate pressure for its depth, residents have pressure-adjustment problems going to the surface. Either way, complex airlocks are required, or a tube to the surface which is vulnerable to the waves. Not only is this a problem for people, but it makes shipping goods difficult as well.
• Limited access to energy sources: The water blocks the sun and waves. While enough sunlight filters down to support plant life in the first couple hundred feet, much of the energy is being absorbed by the water. It might be possible to mount wind turbines on pillars, use currents, or geothermal energy. Still, this makes an already-challenging problem even harder.
• Altitude control. The structure either needs to be located someplace shallow (hard to find in international waters), or needs to have extremely reliable height control so as to neither go up to the surface (and be battered by waves) or down to the depths and be crushed by the pressure.

Note that the positive scenic aspect can be achieved just by having a small undersea portion of a structure which is mostly above the water. (Although many seastead locations will just not be very scenic). While avoiding waves is a huge win, the extra expense and psychological difficulties seem to outweigh that signle large advantage. However, if a cheap construction technique can be created and a good location found (seamount), it could be worth a try. Those interested in more ideas about living underwater should see [Fisher1985, pp. 64-73]