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Hydra
The Hydra is a legendary serpent of great size, endowed with multiple heads, that lived immersed in the swamp of Lerna, near Argos. The number of heads can vary depending on the version of the myth. Diodorus of Sicily attributes a hundred to it, but others give it no more than seven.
The Hydra’s heads were human in shape. Its foul-breathed mouths could dry up wells and springs, poison rivers or lakes. Even when the Hydra was at rest, the venomous air surrounding it could kill both animals and human beings.
Killing the Hydra was no easy feat, since whenever one of its heads was cut off, two would sprout in its place; only Hercules, with the help of Iolaus, was able to accomplish it, as Iolaus cauterized the freshly severed heads with a torch, preventing the beast’s endless regeneration.