Which underworld river is made of fire?

The underworld river made of fire is Phlegethon, also called Pyriphlegethon.

Phlegethon is one of the rivers of Hades in Greek mythology. Its name is connected with burning or blazing, and ancient descriptions portray it as a fiery stream associated with the deepest and most punitive regions of the underworld. In Plato’s Phaedo, it is described as a stream of fire that winds around the earth and flows into Tartarus.

The river is not the same as the Styx, which marked the boundary of the world of the living and the dead and served as the oath-water of the gods. Acheron was associated with sorrow or woe, Cocytus with lamentation, and Lethe with forgetfulness. These symbolic meanings make the rivers easier to distinguish in quiz questions.

Phlegethon appears only briefly in Homer, where it flows toward Acheron, but later Greek and Roman writers expanded its fiery imagery. Dante also used the name in the Divine Comedy, although his Phlegethon is a boiling river of blood rather than the Greek river of fire.