Charybdis

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Historical Background
In Greek mythology, she was a widely feared sea monster who threatened several characters who sailed into her waters (Odysseus, Jason and the Argonauts, et al). She resided on one side of the Strait of Messina* while Scylla resided on the other side of the strait. Together, they represented two dangers where trying to avoid one danger will lead one into the effects of the other danger. This originated the phrase "Between Scylla and Charybdis", the equivalent of the English phrase "between a rock and a hard place" (which is actually believed to derive from the ancient Greek phrase). It is said no ship ever made it through without losses of crew. Charybdis would swallow enormous quantities of water 3 times a day and then spit it back out again, a process which would generate a huge whirlpool. Her neighbor, Scylla dwelled in the caves along the rocky cliffs of the other side of the straight (Scylla was half female, half monster, with 6 long necks extending from the bottom of her torso, each with a dog's head on the end of it with 3 rows of sharp teeth. She also had 12 canine legs and a fish's tail). People tended to prefer going too close to Scylla because while Scylla often takes half a dozen to a dozen crew members, Charybdis can destroy the entire ship. She was originally a sea nymph who was turned into a monster for flooding the land in order to expand Poseidon's kingdom (the sea). Her form tends to be poorly defined and she tends to only be known by the whirlpool she creates. Charybdis means "sucker down". She is believed to be a goddess of the tides, with her well-defined attribute of sucking down and spitting up water 3 times a day representing the three high-low tidal cycles found in a day.

(* This is the traditional interpretation of the location, which has a whirlpool, but it is a minor one, not any great peril. Some scholars believe the real location to be Cape Skilla in northwestern Greece and think ancient writers mixed up the real location with a location near Magna Graecia [the Greek colonies in Italy]).