What is the origin of Phlegethon?

In addition to referring to “a stream of fire or fiery light,” Phlegethon retains its original meaning as the river of fire that surrounded Hades, the underworld, in Greek mythology. Its name literally means “burning” or “flaming” in Ancient Greek. The Phlegethon was one of five rivers in Hades; the others were the Acheron, Cocytus, Lethe, and Styx. While the Phlegethon was known as the river of fire, the Acheron was the river of sadness, the Cocytus was the river of weeping, the Lethe was the river of forgetfulness, and the Styx was the river of hatred. All five rivers coalesced at the center of Hades, and Charon ferried the souls of the dead either along the Acheron or the Styx, depending on the author.