The short answer is that it's a normal pit trap but it's filled with water.
The longer answer is that you should take a look at area 36. UTILITY ROOM in B1 In Search Of The Unknown. There's a link to the adventure on DriveThru RPG on the main jam page but if you can't afford it there are ways of reading old TSR adventures online that are probably illegal and that I definitely don't recommend or endorse in any way.