I apologize for the fact that I'm doing a single dungeon room (or rather, two connected rooms) for this Map Monday, but I was pretty busy this weekend. What I have, though, is a trap you can drop into another dungeon.
A stretch of corridor leads past an ordinary dungeon door (A) that opens inward. Examining the floor in front of the door reveals a pool of water. Behind the door is a room with a floor grate, a rusty dagger, and a solid metal door (B) in the east wall. The metal door has no keyhole and seems to be tightly sealed across the bottom; hinges are on this side of the door, so it should open outward. The door appears to be locked, despite the lack of a keyhole.
Examining the wall beside the door reveals a keyhole behind a loose stone. The keyhole is large enough for a dagger to fit in. Sticking a dagger in it, however, will do nothing unless Door A is closed; if it is closed, characters hear a click from Door B, unlocking it. Turning the knob on Door B, even without pulling it, allows the water filling the room beyond to burst through, possibly knocking down characters and filling both rooms to a depth of four feet. Check for drowning for halflings, dwarves, and prone characters. Once both rooms are filled with water, Door A cannot be opened (water pressure holds it shut.) If the characters open Door B while Door A is propped or spiked open, the water will pour out into the corridor as well, eliminating the drowning risk, but making the floors slippery.
Fumbling around under the water in the second room should discover the two valves and the second floor grate. One of the valves opens the floor grates, allowing water to drain; the other opens a valve on the pipe in the wall opposite the door, allowing a powerful stream of water to flood into the room, filling one room in 3 turns or both linked rooms in 6. While this valve is open, it is theoretically possible to fight the current and crawl or swim up the chute beyond that leads up to a huge reservoir.