The mythological background 
In Greek mythology, Charybdis (or Kharybdis) was once a beautiful naiad and the daughter of Poseidon and Gaia. She assumes the form of a huge bladder of a creature whose face is all mouth and whose arms and legs are flippers. She swallows a huge amount of water three times a day, before belching it back out again, creating large whirlpools capable of dragging a ship underwater. In some variations of the story, Charybdis is simply a large whirlpool instead of a sea monster. Once a lovely maiden, Charybdis was loyal to her father in his endless feud with Zeus. She rode the hungry tides after Poseidon stirred up a storm, directing them onto beaches, destroying entire villages, submerging fields and drowning forests, claiming all in her path for the sea. She claimed so much land for her father's kingdom that Zeus became enraged and changed her into a monster.
In mythology Charybdis lies on one side of a narrow channel. Opposite her is Scylla, another sea-monster. The sides of the strait are within an arrow shot of each other, and sailors attempting to avoid one of them will come in reach of the other. 'Between Scylla and Charybdis' thus means to having to choose between two dangers, either of which brings harm.
The theoretical size of Charybdis remains unknown, yet in order to consume Greek ships the whirlpool can be estimated to about 75 feet 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 seldom dangerous.
References in ancient literature 
The Odyssey 
Throughout the poem, Odysseus is hindered by the efforts of Poseidon and the sea monsters throughout the ocean. Odysseus faced both Charybdis and Scylla in Homer's Odyssey while rowing through a narrow channel. He ordered his men to avoid Charybdis thus forcing them to pass near Scylla, which resulted in the death of six of his men.
Later, stranded on a raft, Odysseus was swept back through the strait to face Scylla and Charybdis once more. This time, Odysseus passed near Charybdis. His raft was sucked into Charybdis' maw, but he survived by clinging to a fig tree growing on a rock over her lair. On the next outflow of water, his raft was expelled. Odysseus recovered it and paddled away safely.
Jason and The Argonauts 
Aristotle's Meteorologica 
Aristotle tells a story of Aesop in conflict with a ferryman and relating to him a myth about Charybdis. She took one gulp of the sea and brought the mountains to view; islands appeared after another. The third will dry the sea altogether.
|Wikimedia Commons has media related to: Charybdis|