Detail of engraving from Locupletissimi Rerum Naturalium Thesauri by Albertus Seba, 1734. In Greek mythology, the Lernaean Hydra was an ancient serpent-like chthonic water beast, that possessed many heads (for each head cut off it grew two more), and deadly poisonous breath and blood. Beneath the waters was an entrance to the Underworld, and the Hydra was its guardian. The Hydra of Lerna was killed by Hercules who cauterized the necks as he cut off the heads. It was the second of his Twelve Labors.