>I think you are grossly under estimating the challenge of living under the ocean. There have been a number of underwater habitats built (both for science and n

>I think you are grossly under estimating the challenge of living under the ocean. There have been a number of underwater habitats built (both for science and nominally living quarters) so there are reports you can read on the experience. I didn't mean to talk about it as if it were a commonplace thing, but compared to settlements on other worlds, it is. I'm not sure what the state of the art is, though as I recall the navy had some habitats in the 70's that they stopped reporting on, so either they closed them down or there's some secret underwater laboratory Deus Ex-style somewhere. >In particular a 'huge asteroid' could not be mitigated by "diving into a trench", water is incompressible by nature and the impact of an asteroid in the ocean would cause a huge over-pressure event in your habitats, where ever they are, resulting in their destruction. Today, such an impact would destroy every submarine in the same ocean where it hit. Well, yes, that is true for submarines. But an undersea settlement would most likely be anchored to the ocean floor and have access to it, mostly for mining, geomethermal power and storage, but you could use it as a shelter in case the main habitat is destroyed. >Gamma rays would be fine, water does make an excellent radiation blocker. That could be achieved on land by building a structure that was inside the middle of a water reservoir. (imagine a large water storage tank with underwater living space in the middle of it.) The ocean has the advantage that it's a single massive body of water, so it's harder to boil all of it off.