Scylla and Charybdis were mythical sea monsters from Greek mythology. Scylla was originally a nymph that was transformed into a six-headed sea monster by the sorceress Circe. Charybdis was a greedy girl who was transformed by Zeus into a whirlpool monster for stealing oxen from Heracles. Scylla and Charybdis lived on opposite sides of the narrow Strait of Messina, making it impossible for ships to pass between them without encountering one monster or the other. The dangers they posed gave rise to the saying "between Scylla and Charybdis," meaning facing two equally dangerous options.