Charybdis was a sea monster who would suck down the waters in the Messina Strait three times per day creating a giant whirlpool and then spit it up again. Whoever was unfortunate enough to be in the wrong place at the wrong time when Charybdis sucked down the water wouldn't be there for much longer. If they tried to go around this fate they would be caught in the mouth of another monster that had six heads known as Scylla.
There were several variations of stories that explain what happened to Charybdis. By one account, she was the daughter of Poseidon and Gaia. Zeus turned Charybdis into a monster after she flooded her father's land and sank it below the sea. The island that Charybdis sank was called Atlantis. When she was cast into the sea below she was given the power to suck in huge amounts of water and spit it out again to create a huge whirlpool and destroy anything around it.
In another story she stole the cattle of Herakles, and for that Zeus struck her down into the sea with a thunderbolt. To keep Charybdis company, Zeus placed another monster across the strait called Scylla. Scylla was a monster that had six heads and with each would eat one of a passing ships' crew. So as a matter of course if you tried to enter the strait you would encounter either Charybdis, who would destroy your entire ship in the whirlpool, or Scylla, who would eat some of your crew.
In Homer's book "The Odyssey", when Odysseus tries to cross the Messina Strait with the cattle of Helius, He is first met by Scylla and each of her six heads. In an attempt to avoid her again, he travels too far towards the...