The Gateway Arch, also known as the Gateway to the West.
Last weekend on our way home from Chicago we enjoyed this view of St. Louis, Missouri and the Arch as we crossed the Mississippi River from East St. Louis, Illinois.
The Gateway Arch was built as a monument to the westward expansion of the United States, and has become the iconic image of the city of St. Louis. It was designed by Finnish-American architect Eero Saarinen and structural engineer Hannskarl Bandel in 1947. It is 630 feet (192 m) wide at its base and stands 630 feet (192 m) tall, making it the tallest monument in the United States, Construction started on February 12, 1963, and ended on October 28, 1965. The monument opened to the public on July 10, 1967. The interior is hollow with trams on either side to take visitors to an observation area at the top. As usual, Wikipedia has an excellent article on the Gateway Arch with good photos of the interior and the view from the top.