Note: This information was taken from TSR's original AD&D adventure, the Temple of Elemental Evil.
For the players: A relatively short flight of stairs leads down to a 20' long walkway, 10' wide. To the north of the walkway is a pit, 20' square and 10' deep. The floor of the pit is slimy and covered with bones and filth. The rough stone walls are pocked with small openings near the floor, but they become smooth and unclimbable four feet above the floor of the pit.

For the DM: If more than 100 pounds of weight are on the walkway while no weight is on the lower stairs, a trap in sprung. The walkway is balanced in a cupped socket, and the weight upon it causes the ledge and seven feet of the wall behind it to tilt sharply down to the north. The smooth stone offers no hold to the hapless victims, each of whom takes 1-6 points of damage from falling into the pit. With the weight gone, the pivoting ledge swivels back to its original position.

At the sound of victims falling into the pit, 24 giant rats rush out from their wall burrows, being trained to attack after that stimulus. Victims lose all shield and dexterity bonuses to armor class in the first round, as well as initiative, but combat is normal thereafter. Each round, 24 more rats rush out and attack, until all 144 have arrived.

Giant Rats (144): AC 7, MV 12"/6", HD 1/2,
hp 4, 3, 2, 1 (36 of each), #AT 1, D 1-3,
SA 8 carry disease, XP 11 (x36), 10 (x36), 9 (x36), 8 (x36)

The rats have no treasure, but 1-12 of each coin type (cp, sp, ep, gp, pp) lie scattered under the litter on the floor. Searchers find one coin per person per round until all are discovered.