Any exits close up, and holes open in the walls. The trapped area begins to rapidly fill with water. The enclosed space will fill in five minutes exactly, drowning any characters within (magic or certain innate abilities may help here). Characters have only that much time to disarm the trap, or somehow break free.
Optional: There is a switch to turn off the trap hidden in plain sight, along with a cryptic clue as to its identity (either in a poem written on the wall, or upon a carving showing the item being used in some innocuous manner).