In Greek mythology, the Lernaean Hydra was the nine-headed monster killed by Heracles during his second labor.
The Hydra lived in the marshes near Lerna, an ancient site in the Argolid. It was described as the offspring of Typhon and Echidna, making it part of a monstrous family that also included the Chimera and Cerberus. Its poisonous breath and blood made it exceptionally dangerous.
When Heracles cut off one head, two more could grow in its place. He therefore needed help from his nephew Iolaus, who cauterized each neck with burning branches so new heads could not regenerate. The central head was immortal in many versions, so Heracles buried it beneath a heavy rock.
The Hydra was the second of Heracles’ twelve labors. Eurystheus later rejected the labor because Iolaus had assisted Heracles, meaning it did not count as a completely solitary achievement. The monster’s name survives in modern language for a problem that produces new difficulties when attacked.