Ships passing through the Straight of Messina between Sicily and Italy had a choice; sail closer to Scylla, or closer to her partner, Charybdis. If they chose Scylla, then she would snatch up six sailors, one for each head, from the deck of the ship. (Any attempt to cheat her would result in her picking up the whole ship and bringing it to her lair.) Charybdis, the whirlpool, would swallow thousands of gallons of seawater and random times and then belch the water back out an unspecified time later. If a ship was caught here at the wrong time, it would be swallowed whole or dragged to the bottom of the sea, entire crew included.
Circe advised Odysseus to sail closer to Scylla. He did so and lost six men, but was able to continue his journey home.