Gateway Arch, in St. Louis, Missouri, is the tallest monument in the United States. The arch, wrapped in stainless steel, rises 630 feet (192 meters) above downtown St. Louis, on the Mississippi River. The popular tourist attraction was built to commemorate the nation's westward expansion during the 1800's. Visitors may take trams up either leg of the arch to an observation room at the top. The arch is part of the Jefferson National Expansion Memorial, which also includes an underground visitor center, theater, and museum.