Lethe

Also known as the Amelēs potamos (river of unmindfulness), the Lethe flowed around the cave of Hypnos and through the Underworld where all those who drank from it experienced complete forgetfulness.

Orpheus would give some shades (the Greek term for ghosts or spirits) a password to tell Hades's servants which would allow them to drink instead from the Mnemosyne (the pool of memory), which was located under a poplar tree.

[2] Some ancient Greeks believed that souls were made to drink from the river before being reincarnated, so that they would not remember their past lives.

The Myth of Er in Book X of Plato's Republic tells of the dead arriving at a barren waste called the "plain of Lethe", through which the river Ameles ("careless") runs.

These two rivers are attested in several verse inscriptions on gold plates dating to the 4th century BC and onward, found at Thurii in Southern Italy and elsewhere throughout the Greek world.

[7] In 138 BCE, the Roman general Decimus Junius Brutus Callaicus sought to dispose of the myth, as it impeded his military campaigns in the area.

The souls that throng the flood Are those to whom, by fate, are other bodies ow'd: In Lethe's lake they long oblivion taste, Of future life secure, forgetful of the past.