Lerna

In classical Greece, Lerna was a region of springs and a former lake located in the municipality of the same name, near the east coast of the Peloponnesus, south of Argos. Even though much of the area is marshy, Lerna is located on a geographically narrow point between mountains and the sea, along an ancient route from the Argolid to the southern Peloponnese; this location may have resulted in the importance of the settlement.

Its site near the village Mili at the Argolic Gulf is most famous as the lair of the Lernaean Hydra, the chthonic many-headed water snake, a creature of great antiquity when Heracles killed it, as the second of his labors. The strong Karstic springs remained; the lake, diminished to a silt lagoon by the 19th century, has vanished.

Source: Wikipedia