Ever heard of caves? There is even monstrous caves called "blue holes" in the Carribean under water that extend miles just under the ocean floor. Did you know that many of the major rivers have rivers under them? The Nile for instance has a subsurface heavy stable current along the bottom and is considered a river of it's own. The point is that on our own little planet we've hardly begun to tap into it's mysteries. Why not have a civilization dwell below the surface? Especially if there was an environmental risk above.