the Pool of Memory in Greek mythology
It is located in the underworld of Tartarus next to a white poplar tree. Its opposite is Lethe, The Pool of Forgetfulness. Souls are to drink from Lethe so they do not remember their lives on earth. Orpheus knew a secret password to get to Mnemosyne, though, which he passed on to a group of ghosts. They dropped the password to Hades's guards and were allowed to drimk from Mnemosyne. This gave them knowledge of past lives, as well as the future.
I drank from Mnemosyne and remembered the answers to the test.
Mother of the nine muses by Zeus. Also known as the goddess of memory.
Mnemosyne is an important but not too mentioned figure in Greek mythology.