Lernaean Hydra

Upon reaching the swamp near Lake Lerna[?], where the Hydra dwelt, Heracles covered his mouth and nose with a cloth to protect himself from the poisonous fumes and fired flaming arrows into its lair to draw it out. He then confronted it, but upon cutting off one of its heads he found that it (or two) grew back, the same happened again upon cutting off a second head; realising that he could not defeat the hydra in this way Heracles called on Iolaus for help. His nephew then came upon the idea (possibly inspired by Athena) of using a burning firebrand to scorch the neck stumps after decapitation and handed him the blazing brand. Heracles cut off each head and Iolaus burned the open stump leaving the hydra dead and taking its one immortal head he placed it under a great rock, and dipped his arrows in the hydra's poisonous blood, and so his second task was complete. In an alternate version, Hera sent a crab to bite his feet and bother him, hoping to cause his death. When Eurystheus found out that it was Heracles' nephew who had handed him the firebrand he declared that the labour had not been completed alone and as a result did not count towards the ten labours set for him.

The term "Hydra" is often used today when referring to a problem that seems to share this sort of characteristic, for example when attempts to suppress a particular piece of information result in it being disseminated even more widely.