Hydra, in Greek Mythology, is an aquatic monster generated by Typhon and Echidna. The Hydra of Lerna appears to be a horrible serpent, with several heads of which one is immortal. It lives in dark swamp that smell putrid. To achieve its ends, Heracles gets help from his nephew who burns the bottom of the heads while his uncle cuts off the others. He buries the last one under a rock.