Troglodyte Cave House.

Ah, the French and their troglodytic dwellings … I can just imagine the conversation when someone first said "look at this troglodyte cave, it’s kind of cozy. I wonder if I can build my house here":

In France there are a lot of troglodytic dwellings, most of them carved back into the soft limestone cliffs. It may have started when they dug cellars for storing wine and produce, and noticed how comfortable it was on a hot day. Somebody said "Why not just move in here?" and a trend was born.

Link | Troglodytes of Doué | Troglyodyte village Rochmenier – Thanks MvT Cracker!