Missouri is a state in the Midwest of the United States of America.
While traveling to the Underworld, Percy Jackson, Annabeth Chase and Grover Underwood stop in St. Louis and see the Gateway Arch. While there, the son of Poseidon is attacked by Echidna and the Chimera and falls off the arch into the Mississippi River 630 feet below.
- Before and during the American Civil War, Missouri was one of the five border states, northern states that allowed slavery who did not secede to form the Confederate States of America. The others were Delaware, Maryland, West Virginia, and Kentucky.