Charybdis

Charybdis was a nymph-daughter of Poseidon and Gaia, whose job, as told in Greek mythology was to flood the lands of her father's underwater kingdom. However, Zeus turned her into a monster, who would suck in water thrice daily, thus forming a giant whirlpool above the cave where she lived.

She lived in a cave at one side of the Strait of Messina, opposite the monster Scylla, and combined, the two posed a grave danger for any passing shipping.