Two of four 338-foot-high intake towers that stand in the deep waters of Lake Mead behind massive Hoover Dam, which straddles the border between Arizona and Nevada in the Black Canyon of the Colorado River. The tower in the foreground shows the current time in Arizona. Called Boulder Dam from the date of its dedication in 1935, the concrete-arch dam was renamed after former U.S. president Herbert Hoover in 1947. Now a popular tourist attraction, the dam's generators provide power for electric utilities as far away as California. Photo by Carol Highsmith.