Looking at houses (on line) in AZ we saw a place with a 200 sq.ft. “fully detached basement”. It was in the middle of the yard of an otherwise unremarkable house. I asked our agent about it, assuming it was an old 1950s bomb shelter. He opined that it was a storm cellar (he is from the Midwest). Storm cellar in Phoenix? I think I might have a new understanding of this modern home feature.