Which underworld river is the river of wailing?

The underworld river known as the river of wailing is Cocytus.

Cocytus comes from the Ancient Greek word for lamentation or mourning. In Greek mythology it is one of the rivers surrounding Hades, the realm of the dead. It flows into the Acheron, another important underworld river, and is associated with the cries and grief of the dead.

Greek traditions commonly name five underworld rivers: Styx, Acheron, Phlegethon, Lethe, and Cocytus. Their meanings and roles overlap across authors, so later summaries do not always describe the geography identically. Cocytus is distinguished by its association with wailing, while Lethe is linked with forgetfulness and Phlegethon with fire.

Cocytus also appears in later literature. In John Milton’s Paradise Lost, its name is explicitly connected with loud lamentation. Dante uses Cocytus differently in the Inferno, portraying it as a frozen lake at the lowest circle of Hell, where traitors are imprisoned. That icy Dantean landscape is a later literary development, not the standard Greek image of a mournful river.