Ref. my "spare" fallout shelter, the guy who owned my home before I did was an old-school prepper. He was a WWII vet who built a fallout shelter in the fifties. I found his notes. He actually built it at night so that the neighbors wouldn't know he had it. It's got solid concrete walls and an 18" thick roof with several feet of soil on top of that. I like it, but it is small and right now that end of the house is being slowly renovated so it isn't in use. I have an eventual plan for it, but that's down the road.