The next afternoon, Percy returns to the Saw Mill River in Yonkers with Annabeth, Grover, and Iris’s staff. Annabeth brings practical gear, Grover brings snacks and panpipes, and the three enter the tunnel in search of cleaner water upstream because the creek bed is too filthy to wash the kerykeion properly.

Inside the tunnel, the group finds glowing moss and briefly relaxes by drawing light patterns on the walls. The tunnel then opens into a vast underground cavern where the River Elisson winds through yellow grass, ash-colored trees, and blue-green light. The water is cleaner, but the shallows are crowded with many snake-like monsters bathing in it.

Because Grover smells monsters throughout the cavern, Annabeth suggests sneaking through the tall grass rather than fighting. Grover leads the way, showing more confidence than Percy expects, and the group reaches a boulder island. From there they see that snakes occupy nearly every accessible part of the river, but Annabeth spots a cleaner, more dangerous pool near the northern waterfall that the snakes cannot easily reach.

Grover proposes a risky plan: Percy and Annabeth will head for the cliffs while Grover watches from the boulder. If the snakes notice them, Grover will distract the reptiles with his panpipes and draw the danger toward himself, buying Percy time to wash the staff. Percy worries about using Grover as a decoy again, but agrees because Grover is determined and capable.

Percy and Annabeth make their way through the sharp, noisy grass to the waterfall ledge. They find a clear pool below, but Percy hesitates because there is no obvious way to climb out and he remembers Iris mentioning an angry river god. Before they can make a careful plan, Grover’s distant panpipes begin playing “Union of the Snake,” signaling that the snakes are on the move, so Annabeth pushes Percy into the pool with the staff.