I'm not sure if this is the right category, but when I was really little my dad would tell me that fraggles (they're Jim Henson muppets, if you don't know) lived in our basement behind the furnace. I don't think I ever went to search for them, but I did believe him.
A childhood belief about basements

others believed things like this
- My sister told me that Fraggles lived in our basement. I used to sit down there forever so I could see them. — Suzie
- I had a little hole in the wall under my bed when I was little and I thought that Fraggles lived in there.....— Tina
- when i was little i used to believe that darth vador lived in my basement, and i was scared to go down there.— Jilli
- My grandma told me and my sisters that the boogie man lived in her basement and if we cried he would get us.— Paulette
- The muppets were REALLY inside the T.V.!Anon
- i used to think that when i went to sleep gonzo from the muppets would suck me up through his nose and eat meAnon