In Greek mythology, Scylla was a sea monster, with twelve feet and six heads, each with three rows of teeth. She lived underneath a rock at one side of the Strait of Messia, opposite the whirlpool of Charybdis. This proved to be a hazardous passage for passing ships, as they would be sucked into the whirlpool if they sailed too far from Scylla, but ran the risk of having their crews eaten if they sailed too close to her. She ate six of Odysseus' crew, as they sailed past. The concept of being "between Scylla and Charybdis" is considered to be one of the origins of the saying, "between a rock and a hard place."
She was originally a nymph, a daughter of Phorcys, but was transformed into a monster by the jealous Circe, who poisoned the pool where Scylla bathed in order to win the sea-god Glaucus for herself.