Charybdis

The name of a sea monster believed to live under a small rock on one side of a narrow channel. Opposite her was Scylla, another sea monster, that lived inside a much larger rock. The sides of the strait were within an arrow-shot of each other, and sailors attempting to avoid one of them would come in reach of the other. "Between Scylla and Charybdis" therefore means to having to choose between two dangers, either of which brings harm. Three times a day, Charybdis swallowed a huge amount of water, before belching it back out again, creating large whirlpools capable of dragging a ship underwater. In some variations of the story, Charybdis was simply a large whirlpool instead of a sea monster.