Symplegades

The Symplegades, also known as The Cashing Rocks, are an entrance into The Sea of Monsters.

The Sea of Monsters
Annabeth Chase suggests using the Clashing Rock to enter the Sea of Monsters, instead of going between Scylla and Charybdis. Clarisse La Rue rebuffs the idea, saying, "I can't blow rocks apart with my cannons. Monsters, on the other hand..."

Percy Jackson's Greek Heroes
Phineas tells the Argonauts how to get past them.