Illustration on a postcard of giant tunnels creating diversions for the Colorado River during the construction of Hoover Dam. The caption reads: "Diverting the Colorado River through giant tunnels."

Postcard with an illustration of Hoover Dam. The caption reads: "Boulder Dam, as it will look when completed, Colorado River."

Illustration of Boulder Dam, later called Hoover Dam, construction. Caption reads: "A close-up, showing construction detail, Boulder Dam, Colorado River."

Postcard with an illustration of Boulder Dam view from Lookout Point. Boulder Dam was later renamed Hoover Dam.

Guest house and main building of Scotty's Castle. The National Park Service bought the private property in 1970 and it is now a part of Death Valley National Park. Site Name: Scotty's Castle (Death Valley, Calif.)

Postcard of watchtower at Scotty's Castle. Scotty's Castle was bought by National Park Service in 1970 and it is now a property of Death Valley National Park.

A picture of an unidentified worker standing in front of Las Vegas and Tonapah Stations in Goldfield, Nevada.