Here's what happens:
* The Arch's Connection: The Arch, a monument to westward expansion, is actually built on a spot where the ancient gods of the Underworld once reigned.
* The Entrance: Percy Jackson and his friends, Annabeth Chase and Grover Underwood, discover that a giant, pulsating "eye" has appeared at the top of the Arch. This "eye" is actually the entrance to the Underworld.
* The Threat: The giant, one-eyed cyclops, Tyson, is imprisoned beneath the Arch, and his imprisonment is what keeps the Underworld from overflowing into the mortal world.
* The Escape: During the climax of the novel, Percy and his friends must face the god Ares and his allies. Ares unleashes a massive storm that cracks the Arch, threatening to release Tyson and unleash the Underworld upon the city.
* The Solution: Percy, using his powers, manages to patch up the Arch and restore the barrier between worlds, preventing the Underworld from flooding out.
The St. Louis Arch is a symbol of the clash between the ancient world and the modern world. It represents the hidden dangers that lurk beneath the surface of everyday life.