Beyond lay Erebus, which could be taken for a euphonym of Hades, whose own name was dread. There were two pools, that of Lethe, where the common souls flocked to erase all memory, and the pool of Mnemosyne ("memory"), where the initiates of the Mysteries drank instead.
From Wikipedia, 4/30/2018
The Mysteries refers to a cult of Demeter.
The pool of Mnemosyne is not often mentioned and I can't find much else on it.
Can information about it be expanded upon?