The Hydra of Lerna was one of the many horrific offspring of Echidna and Typhon. Set to guard the entrance to the underword beneath Lerna Lake, the Hydra was feared for its deadly poison, which was exuded from its blood, bite and breath. In addition, it was long believed to be impossible to kill, as for each of its heads that was removed, two more would spring up in its place. It was the hero Hercules who finally slew the beast by cauterizing the stump of each head as soon as it was removed. However, one head was immortal, and continued to writhe after being severed. This head Hercules trapped under a boulder, where it is said that its poison leaked out and fouled nearby springs.