ca. 1931-1936, Boulder City, Nevada, USA — Massive steel bar columns rise in the construction of Boulder Dam. There is as much steel in the dam as is in the Empire State Building and enough concrete to lay a highway from California to Florida. Now called Hoover Dam, it is one of the world’s largest dams, holding back the waters of the Colorado River between Arizona and Nevada to create power and Lake Mead. Nevada. — Image by © CORBIS