Trap door

A trap door is a hidden door located in the floor. When triggered the door falls open and creates a pit that drops anything over it onto a random spot in the dungeon level below.

One technique of dealing with this problem is to go down any stairs you encounter and then go up. This way, if you should ever fall down to that level you will know where the exit is. A high dexterity status will reduce your chances of falling down the pit should you accidentally trigger a trap door.