A monster in Greek mythology. Charybdis (pronounced "kar-IB-dis) had once been a beautiful nymph, a daughter of Poseidon and Gaia. She caused floods in order to expand her father's kingdom, until she was transformed by Zeus into a horrifying sea monster. She was, of course, one half of the Greek hip pop duo Scylla and Charybdis, though, like Andrew Ridgely, she tended to be overshadowed by her more visible partner.
Scylla and Charybdis lived on opposite sides of the narrow Strait of Messina -- Scylla was the six-headed serpentine beast in a cave high on a cliff, and Charybdis was the rarely-seen one who lived in a cave down at the water's edge. Three times a day, she would suck down a huge amount of water, then spit it back out again, forming a terrific whirlpool. While Scylla could reach down and snare a half-dozen sailors from any passing ship, Charybdis was considered the far more dangerous threat, as she could effortlessly destroy an entire ship all by herself. Not even the gods were capable of saving a ship that got trapped in Charybdis' maw. Together, Scylla and Charybdis were colossally deadly, and the Strait of Messina was considered completely impossible to navigate because of them. Only the Argonauts had been able to avoid both of the dangers, and that was because they had the guidance of Thetis, one of the Nereids.
In "The Odyssey", Odysseus was warned by Circe that his best bet for getting past Scylla and Charybdis was to sail as rapidly as he could and to keep to Scylla's side of the strait to avoid losing his ship and all his men to Charybdis' whirlpool. Odysseus was able to avoid Charybdis and get through the strait, but at the cost of six of his men, who were carried away by Scylla.
Research from http://www.pantheon.org/articles/c/charybdis.html, Bullfinch's Mythology, and http://homepage.mac.com/cparada/GML/Charybdis.html