Charybdis was a sea-monster that swallowed vast amounts of water, causing whirlpools in the sea, which could swallow nearby ships. Charybdis stay near to Scylla, another monster on a cliff, so that ships that wished to pass by needed to choose between encountering Charybdis or Scylla. Odysseus, choosing between the two, feared Charybdis the most, so encountered Scylla.