Monument in St. Louis to commemorate westward expansion.
Free-standing arch in the form of an inverted catenary, a curve that has the unique property of having no bending moments present anywhere along the curve when loaded by gravity - thus the arch can be self-standing without being extremely massive (think of the Arc de Triomphe).
[link|http://www.nps.gov/jeff/|http://www.nps.gov/jeff/]