Trap door

A trap door or trapdoor is a door in the ground or in a floor, that one enters vertically. Some can open when someone is standing on it. People typically put small doors disguised as something normal to scare a person falling into a trapdoor. They can be used for malicious purposes like murder. However, they can be used to hide things from other people. 
Trap doors are popular in mystery themed movies. The victim who falls down the trap door often is met with danger or a hidden treasure. A standard cliché deals with the victim finding himself in an alligator filled swamp (most famously seen in "Scooby Doo" cartoons).