The Gateway Arch, which sits on the Mississippi riverfront, was constructed in the early 1960s as a monument to the American pioneers who settled the western frontier.
The giant stainless steel arch rises 630 feet (192 m) and is the tallest man-made monument in the United States.
Visitors can ride trams through the arch to a viewing area at the top, and on a clear day they can see the horizon 30 miles (48 km) away.
The Museum of Westward Expansion lies beneath the arch.
The foundations of the arch had to be sunk 60 feet (18 m) into the ground.
The arch was built to withstand earthquakes, which can occur in the area.
It was also designed to withstand high winds and can sway up to 18 inches (46 cm).