Henry Jo Kaiser signs a contract on March 11, 1931 for the federal government to start building the Hoover Dam with the help of contractor Six Companies Inc. Six Companies Inc. was a joint venture between Bechtel Corporation, Kaiser Industries, and several smaller construction companies across the West. The project started on April 20, 1931 and finished on March 1, 1936. It cost $49 million and is the second highest dam in the country. View photo »

A map of Hoover Dam shows how the concrete dam fits into Black Canyon, including the powerhouse where water churning through turbines in 17 power generating units produces electricity for Nevada, California and Arizona. The dam was built to protect farmers in Arizona and California as well as produce electricity from the raging Colorado River. View photo »

This schematic shows Hoover Dam and its power plant, diversion tunnels, spillways and penstocks on both the Nevada and Arizona sides. The dam restrained the Colorado River in Black Canyon. View photo »

A diagram of the visitors center at the Hoover Dam shows the site from the northwest. The visitors center is completed on June 21, 1995 at a cost of $87.9 million, almost double the cost of the dam itself. View photo »

A diagram of the visitors center at the Hoover Dam shows the site from a southwest perspective. The Hoover Dam has become a pop culture icon having a large role in the movie "Fools Rush In" and "Transformers." In the original movie "Superman," a man-made earthquake causes the Hoover Dam to burst open. View photo »

A model of the Hoover Dam in December 1989 shows the visitors center and parking garage. In 1988 and 1989, U. S. Highway 93 on the Nevada side of the dam was realigned and a new bridge built to carry traffic over the pedestrian walkway, linking the parking garage and the visitor center. An $87.9 million visitors center was completed on June 21, 1995. View photo »