The reward money from the poodle’s return pays for train tickets as far as Denver. The three spend the next two days on the train. Percy mumbles in his sleep, and when Annabeth asks who he was talking to, Percy finally explains the dream about the presence in the pit. Annabeth says it doesn’t sound like Hades, but Percy argues it must be because the presence offered Percy his mother back. Annabeth warns Percy against making deals with Hades. When Percy asks what she’d do if it was her dad, Annabeth responds, “I'd leave him to rot” (200).

The train stops in St. Louis, and Annabeth insists they go to the top of the Gateway Arch. She wants to be an architect so she can design monuments like it someday, and she refuses to miss seeing it. The whole way up, Percy feels like something isn’t quite right but can’t figure out what it is. They board the arch’s elevator with a large woman and her little dog. On the way down, there’s no room for Percy in the elevator. He sends Annabeth and Grover down and finds himself waiting with the woman and dog. The dog morphs into a chimera, and the woman reveals herself as Echidna (mother of monsters).