Scylla and Charybdis
In classical mythology, Scylla was a horrible six-headed monster who lived on a rock on one side of a narrow strait. Charybdis was a whirlpool on the other side. When ships passed close to Scylla's rock in order to avoid Charybdis, she would seize and devour their sailors. Aeneas, Jason, and Odysseus all had to pass between Scylla and Charybdis.