Lerna was a region in the Peloponnese in Greece, which was the backdrop for one of the Labours of Heracles in Greek mythology. It was the place where one of the best known mythical monsters dwelled, the Lernaean Hydra. It was a water snake that had nine heads; eight of them were mortal, but as soon as one was cut off, two more would appear. The ninth head was immortal and could not be slain by normal means. Slaying the Lernaean Hydra was the second labour that King Eurystheus gave to Heracles, as was demanded by the goddess Hera.
Lerna was a region that had lots of springs and swamps, and was also one of the entrances to the Underworld. In this area, the Lernaean Mysteries were held as a tribute to the goddess of nature, Demeter. It was also here that the god Poseidon slept with Amymone, daughter of Danaus, to whom he revealed the secret of the Lernaean lake.