When I was very little, we lived in a house like that, except not in bad condition. Long story, but we were quite wealthy, and then very suddenly, never again. Lol.

Anyway, my brother and I decided it would be fun to sled down the stairs one day, and toward the bottom of the stairs one of us hit one of those little metal strips that held the carpet down on the stairs with the edge of the sled. When my dad was fixing it later they discovered that the stairs lifted up Munsters style, and there was an entire room down there, under the floor, with a tunnel that lead God knows where.

Turns out it wasn't satanists, but was in fact part of the underground rail road, and was used by folks smuggling slaves up north. Ridiculously cool to thing to have in your house