In the original Caverns of Thracia, there is a trap door marked on the map.
It is next to the statue in room 20, where the tribesmen prepare for their sacrifices.
Where does this lead to?
Do the tribesmen know about it?
According to the text of the module, it doesn't specify where it leads. (See Description of the Purification Hall).
However, if you want to extrapolate it, you can judge about where it would lead by comparing level 1 and level 2. The trap door (24) has a description that it leads to the pool below the rapids, and while they don't exactly line up, you could fudge that the trap door in the Hall drops them actually into the rapids.