The Gateway Guardians are based in "The Gateway to the West" St. Louis, MO. The Gateway Arch is actually a site of mystic power and has attracted much weirdness over the years. Unbeknownst to most people (including the players ) the Arch was designed by a slightly insane engineer who dabbled in magic. He used special materials in the construction that he thought would help attract mystic energies. He was correct. However, he was incorrect in thinking that he could control said energies and he died, of what most people think was a heart attack, right after it's completion.
Speaking as someone who lives in southern Illinois, don't go breaking too much stuff.
SAW