St. Louis is a city in the state of Missouri.
Percy Jackson, Annabeth Chase and Grover Underwood arrive in the city. The daughter of Athena urges her companions to see the Gateway Arch. Grover smells monsters, but since they are underground, he says it's just a bad smell. As Percy is separated from the group by taking the elevator on his own, he ends up battling Echidna and Chimera. He then jumps through a hole in the floor, and lands in the Mississippi River where he meets a Nereid.
- St. Louis is named after Louis IX of France.