St. Louis, MO

There is also a visitor center, an IMAX type movie theater, and museum at the bottom of the Arch. Rather it is all underground in a large subterrainian complex directly beneath the Arch. This is the mural along the entrance to one of the theaters.
In order to finish the Arch they had to use a temporary and movable bridge that kept the two halves from falling inward. You can see this device depicted in this model of the arch just before the final block was inserted.