I reckon it is a great idea, if you do a search on cave houses you will find many of them overseas.
Wonder what the cave houses in Cooberpedy cost to build?.
I will do an underground carpark one of these days, maybe an underground house, I like the idea.
What if you put a series of shipping containers under the ground say only around ten feet or so down, could be worth looking into in bush fire prone areas, great insulation also, probably need to be on the side of a hill or drainage/damp could be an issue.