Charybdis was a sea monster, later rationalized as a whirlpool and considered a shipping hazard in the Strait of Messina.
The sea monster Charybdis was believed to live under a small rock on one side of a narrow channel. Opposite her was Scylla, another sea monster, that lived inside a much larger rock. [Odyssey, Book XII] The sides of the strait were within an arrow-shot of each other, and sailors attempting to avoid one of them would come in reach of the other. To be "between Scylla and Charybdis" therefore means to be presented with two opposite dangers, the task being to find a route that avoids both. Thrice a day, Charybdis swallowed a huge amount of water, before belching it back out again, creating large whirlpools capable of dragging a ship underwater. In some variations of the story, Charybdis was simply a large whirlpool instead of a sea monster.
The theoretical size of Charybdis remains unknown, yet in order to consume Greek ships the whirlpool can be estimated to about 23 meters (75 ft) across. Charybdis has been associated with the Strait of Messina, off the coast of Sicily and opposite a rock on the mainland identified with Scylla. Were Charybdis to be located in the Strait of Messina it would in fact have the size to accommodate the whirlpool. A whirlpool does exist there, caused by currents meeting, but it is dangerous only to small craft in extreme conditions.
A later myth makes Charybdis the daughter of Poseidon and Gaia and living as a loyal servant to her father.
Charybdis aided his father Poseidon in his feud with her paternal uncle Zeus and, as such, helped him engulf lands and islands in water. Zeus, angry for the land she stole from him, captured and chained her to the sea-bed. Charybdis was then cursed by the god into a hideous bladder of a monster, with flippers for arms and legs, and an uncontrollable thirst for the sea. As such, she drank the water from the sea thrice a day to quench it, which created whirlpools. She lingered on a rock with Scylla facing her directly on another rock, making a strait.