Charybdis is a sea monster in Greek mythology who swallows huge amounts of water three times a day and then spouts it back out again, forming an enormous whirlpool. She lay on one side of the narrow Strait of Messina between Sicily and Italy. On the other side was Scylla, another sea-monster. The two sides of the strait are within an arrow's range of each other, so close that sailors attempting to avoid Charybdis will pass too close to Scylla and vice versa. The Argonauts were able to avoid both dangers because they were guided by Thetis, one of the Nereids. Odysseus was not so fortunate; he chose to risk Scylla at the cost of some of his crew rather than lose the whole ship to Charybdis.
The myth of Charybdis is based upon an actual whirlpool that forms between Scicily and Italy due to the ocean currents there.