For today's mythology post, I turn your attention to Scylla and Charybdis, two monsters who live on either side of the Strait of Messina in Italy. They're positioned so that it's pretty much impossible not to run into one or the other. At one point, both of these hideous monsters were nymphs. The sea god Glaucus was in love with Scylla, but she didn't return his love. When Glaucus sought the assistance of the witch Circe, she fell in love with Glaucus herself, and transformed Scylla into a monster to get her out of the way. In another version of the myth, it's Poseidon who was in love with Scylla, and his wife Amphitrite who turned her into a monster. Her monster form is described as having six heads, each with three rows of teeth, and a body made up of various animals. She is unable to move, and devours sailors who try to pass by.
As for Charybdis, she was a daughter of Poseidon who flooded lands for her father. Zeus was angry over how much land she was taking, and so transformed her into a monster as well. Her form was basically that of a whirlpool, which would suck in ships and then spit them out again.
"Between Scylla and Charybdis" became a phrase that meant a choice between two undesirable options, and is said to have eventually morphed into "between a rock and a hard place." Not that this expression is exactly parallel, since Scylla would almost certainly be the rock, but I'm not sure a whirlpool would count as a hard place. Well, maybe if you use "hard" in the sense of "difficult." I don't know.