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).
Ideas on cryptic clue for switch in plain site?
On solar day 12.2019 by Anon
@Anon I think you'd want to adapt to what your players are looking for. If somebody says they want to do a perception check for an-off switch, then I'd reveal an off-switch on the underside of a shelf or inside a nook or something. If somebody say they want to disable the trap, then I'd make it more complex or less-obvious, like a stone that must be rotated such that the symbol of the local water god (I'm a fan of Slyd from the Pegana pantheon) is pointing up or down or whatever. Or if your players are just outright panicking, you could make the switch exist on the ceiling, such that as long as they can tread water, they'll reach the switch as the room fills, and can deactivate it JUST IN TIME.
On solar day 12.2019 by Klaatu