In the Twelve Labors of Hercules, which monster lived in the marshes near Lerna?

The Lernaean Hydra lived in the marshes near Lerna.

The Hydra was a water monster described as a serpent with multiple heads. It inhabited the region around Lake Lerna in the Argolid, where its presence made the surrounding area dangerous. Hercules was ordered to kill it as his second labor.

The monster’s most famous feature was its regenerative ability. When Hercules severed one head, new heads could emerge from the wound. Iolaus helped by burning the neck after each decapitation, preventing further growth. Hercules also buried the Hydra’s immortal head beneath a heavy rock in the traditional story.

The Hydra’s blood or venom later became a deadly substance associated with Hercules’ arrows. That episode belongs to later myths involving the centaur Nessus and Hercules’ wife Deianira, rather than being the central action of the Hydra labor itself. Lerna, therefore, connects both the monster’s habitat and the name used for it.