The Hydra was an enormous serpent with nine heads—or as many as fifty or one hundred—one of which was immortal. Its parents were the giant Typhon and Echidna, half maiden and half serpent, the so-called “mother of all monsters.” The Hydra’s haunts were the marshes of Lerna, near the city of Argos.
The second of Hercules’s Labors was the slaying of the Hydra. But each time that Hercules would cut off one of its heads, two would grow in its place. Not only this, but a giant crab that kept the Hydra company appeared and joined the fray. Hercules’s clever nephew and companion Iolaus proposed a remedy: the moment Hercules severed a head, Iolaus would cauterize the stump with a firebrand. This the two did until only the Hydra’s immortal head remained. Hercules then cut off this head, and after burying it beneath a boulder, dipped his arrows in the Hydra’s lethal venom. Both the Hydra and the crab, which Hercules had also slain, were placed in the heavens by the goddess Hera as constellations.