Note: This information was taken from TSR's original AD&D adventure, the Temple of Elemental Evil.
For the players: This oddly shaped chamber is bisected by a curving wall of well-dressed, mortared stones. The wall is about four feet tall, and to the south side of it is water, high enough to almost flow over the top. Water trickles in from several places along the east, south, and west walls; overflow gravitates to floor drains even as you watch.

For the DM: Careful examination of the walls and floor within the cistern discovers that the stones are covered with some form of algae, apparently quite slimy and presumably slippery. The floor of the cistern immediately south of the retaining wall is four feet lower than that in the room, and slopes down to the south about two feet for every five; thus, at the far south end, the cistern is about 15 feet deep. The overflow drains carry the water to the dungeon level below.

Anyone trying to stand on the slippery cistern floor slips on the slimy growth (no saving throw) and slides to the lowest part of the cistern, drowning in 3-5 rounds unless rescued or light enough to float. In that deepest portion is a large round slimecovered object, which is a shield +2.