"As doth the billow there upon Charybdis, That breaks itself on that which it encounters"

(Canto 7, lines 22-23)

Together with Scylla, Charybdis was a mythical sea monster found in the Homeric poems, among others. They were later rationalized as a rock (Scylla) and a whirlpool (Charybdis) in the Strait of Messina in southern Italy, where the Ionian and Tyrrhenian seas meet.