Wouldn't a self sustaining underwater colony protect us from all but the worst asteroid impacts? If you go even 100m down surely you would be protected from pretty much everything but the earthquakes?
Also, 1 ATM of pressure is a lot easier to build for than 10 or 100.
Its true the that pressure difference between vacuum and sea level is much greater than between sea level and deep underwater. However you have the advantage of not having to lift everything up to orbit. You you can build a really heavy steel tank using relatively 'simple' construction techniques, then just tow it out to position, before sinking it and anchoring it to the seabed.
The more I think about it, the more it sounds like like a self-sustaining undersea colony would be a affordable and useful way both ensure species Survivability and act as a prototype for a space colony.
All birds are dinosaurs (in the same sense that humans are apes). Hell, you've probably eaten a dinosaur egg this week. And the water-based dinosaur descendants are very well known and the inspiration for how dinosaurs now look in musea, the crocodiles, when we're not actually sure whether crocs or chickens looked more like land based dinosaurs.
http://www.smithsonianmag.com/science-nature/Dinosaurs-Livin...
Large animals go extinct on a regular basis. The reason animals become big is for efficiency, and so often a prelude to extinction (of that particular branch). A rat is much more efficient (work/energy) than an insect. A cat is (a lot) more efficient than a rat or a mouse. Cows and other large mammals are unbeatable when it comes to the amount of energy a kilogram of cow tissue requires to stay alive. But there's usually a reason animals become efficient, and that reason is that their methods of gathering energy are actually becoming unusable, and so energy available to the species is lowering. So the species reduces in numbers, and the physical size of individuals grows. Increased efficiency only happens out of necessity, and if it can't turn the tide, the next step is extinction ... And yes, humans definitely count as large animals. I wonder if we're an exception.
They're completely useless, actually: they're in insulating rock with no ability to conduct away heat. As soon you try to exploit a temperature gradient, you destroy it.