The Gateway Arch monument in St. Louis, clad in stainless steel and built in the form of a flattened catenary arch, 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 Gateway Arch National Park, which was known as the Jefferson National Expansion Memorial until 2018, and has become an internationally famous symbol of St. Louis and the Midwest.