1 Adam: What about the Lethe, the waters of which are reputed to wash away all of one's memories?
2 Charon: You want me to take you there?
2 Adam: Yeah!
3 Charon: You have something you wish to forget?
4 Jamie: I wish to forget what Adam's about to suggest...

The Lethe is one of the mythical rivers of the Greek Underworld.

The ancient Greeks seem to have been pretty keen on defining actual, specific geography for places that nobody had ever been.

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2018-11-18 Rerun commentary: In fact, the ancient Greeks defined not one, not two, but six different rivers of the Underworld:

1. The Styx: The main river of the Underworld, also known as the river of hatred. It encircles the Underworld of Hades seven times^[1]. It eventually runs into a great marsh at the centre of Hades. It has the power to confer invulnerability, as in the story of Achilles, whose mother dipped him in the river Styx, holding him by his heel to do so - thus resulting in his one weak spot, the original Achilles' heel.
2. The Lethe: The river of oblivion, or forgetfulness. It has the power stated in the comic above, of making people forget everything they knew. This river also encircles Hades^[2].
3. The Acheron: The river of pain, or woe. This is most often the river cited in mythology that Charon, the ferryman of the Underworld, rows souls across to take them from the land of the living to the land of the dead^[3].
4. The Phlegethon: The river of fire. This river runs parallel to the river Styx^[4].
5. The Cocytus: The river of lamentation, or wailing. This river - guess what? - encircles Hades^[5].
6. Oceanus: This river is best known for being effectively the sea, encircling the entire mortal world. And because it encircles the mortal world... it separates the mortal world from the Underworld! Yes, this river also separates Hades from the mortal world^[6].

[1] Which is some truly wacky geography. The river goes around the Underworld seven times... so presumably there are seven copies of the river between Hades and the outside world, and you'd need to cross all seven of them to go from one place to the other. I guess it's some sort of enormous spiral?

[2] Sure, why not?

[3] That's another river that encircles Hades, separating it from the land of the living.

[4] Which means the Phlegethon must also encircle the Underworld, not just once, but seven times like the Styx. We're now up to sixteen watercourses that encircle Hades and separate it from the land of the living.

[5] Seventeen. And counting.

[6] The ancient Greeks clearly loved their fantasy geography. But were they any good at geography? Debatable.