The Hydra was a  strange creature which had many snake-like heads. One of the heads was immortal. The other heads grew back if they were cut off.

It was also called the Hydra of Lerna because it was said to have lived near Lake Lerna.

One of Hercules tasks was to kill the Hydra.  He cut off the heads and burned the necks so that the heads could not re-grow. Then he buried the  immortal head under a huge rock.

The Hydra of Lerna was dead.