What is the Spring of Lethe?

The Lethe River ran through the plain of Lethe in Hades. Also known as the stream of oblivion, the river flowed around the cave of Hypnos where the murmuring waters caused drowsiness. The shades of the dead were required to drink the water in order to forget their earthly life. Lethe is frequently used as a metaphor for the Underworld in general.