[bruce511]

Building under the sea at any kind of scale falls down quickly as soon as you compare it to say building it on land.

In other words, from cost, to sustainability, to environment, to trade, to defence, to literally whatever criterion you want, living on land is better than living under the sea.

And alas it would not really increase survivability of ELEs. (inside mountains is likely better for that.)

The only reason to colonise Mars is to mitigate an ELE, and its a pretty poor mitigation at that - Mars is more likely to experience an ELE than we are, all it takes is a bit of machinery to fail.

Which begs the question of course, why do we care? If an ELE happens, then it happens. Bummer.