Scylla and Charybdis are two sea monsters of Ancient Greek mythology who were situated on opposite sides of the Strait of Messina between Sicily and Calabria, in Italy. Personifying a natural rock formation and a whirlpool, they were located in close enough proximity to each other that they posed an inescapable threat to passing sailors; avoiding Charybdis meant passing too closely to Scylla and vice versa. They have become a proverbial metaphor synonymous with "between a rock and a hard place," "between the devil and the deep blue sea," "Catch-22," etcetera.