“Forgetfulness,” in the Greek underworld, or the Water of Oblivion, is a spring rising to the River Lethe. According to mystery cultists, the spring of Lethe can be found under the a white cypress tree and is the first place a newly dead soul visits. The soul, thirsty from the voyage would be tempted to indulge in the waters of forgetfulness. The ever vigilant soul, should train to learn an endurance to thirst and thus receive enlightenment. The enlightened soul would thus instead seek the spring of Memory called Mnemosyne.
“Thou shalt find to the left of the house of Hades a spring, and by the side thereof standing a white cypress. To this spring approach not near. But thou shalt find another, from the lade of Memory cold water flowing forth, and there are guardians before it.”
In classical and Gnostic texts, the exact location of Lethe is a mystery…but one is to made to believe that after preparing to enter the dark pit, one will learn his fate through “things seen” or “things heard.” Thus he has to drink of the waters of Lethe, in order to achieve forgetfulness of all previous life memories so as to achieve a new life in the underworld.
Classical writers claim Lethe as of the five major rives of the underworld, including Acheron, Cocytus, Phlegethon, and Styx.
Source: Book of Myths and Secrets