The Gateway Arch is a 192 m monument in St. Louis, Missouri. Clad in stainless steel and built in the form of a flattened catenary arch, it is the tallest man-made monument in the United States, and the world's tallest arch. Built as a monument to the westward expansion of the United States, it is the centerpiece of the Jefferson National Expansion Memorial and has become an internationally famous symbol of St. Louis.