I believed the shark from Jaws lived under my bed at night. To avoid being bitten by this large ocean-roaming eating machine, I constructed elaborate Rube Goldberg-esque systems to turn off the light switch across the room from my top bunk. Ultimately, the systems would fail (scotch tape does weaken over time). I would be petrified about jumping as fast as I could to the light switch and back to the bunk. I had bruises on my stomach for a full year from missing the bunk -- which, I suppose, was better than being attacked by a shark in my bedroom in upstate New York's dairy and grape-growing region.
A childhood belief about under my bed

others believed things like this
- A friend believed that they used a tame shark during the filming of "Jaws".— Wendy
- I used to believe that if you jumped off the top of a bunk bed, you'd magically gain superpowers.Anon
- I used to believe that there was an off switch to the ocean so that it could go to sleep at night.Anon
- I used to belive that if we could find the light switch that controlled the sun, we could control the world. Anon
- I used to think that a "pool shark" was an actual shark that lived in a swimming poolAnon
- I used to believe that a person could fix ANYTHING with Scotch tape.— Felix