One of the offspring of Typhon and Echidna. The Hydra was a multi-headed water serpent that breathed poisonous gas and had toxic blood, and every time one head was cut off, two more grew back in its place. The Hydra dwelled in the Spring of Amymone in the swamp or lake of Lerna near the Peloponnese, beneath which was said to be an entrance to the Underworld. The Hydra was killed by Heracles as his second labor for Eurystheus during a battle in which Heracles' nephew Iolaus provided aid by cauterizing the neck stumps after Heracles cut each head off, preventing additional heads from growing back. After killing the monster, Heracles dipped his arrows in the Hydra's blood; the poisoned arrows were later used against the Stymphalian Birds, Geryon, and the centaur Nessus.