12 May 1933, USA — Original caption: Near Las Vegas: 4-Ton Dynamite Blast Set Off At Hoover Dam Site. A mountain of dirt and rock rises skyward after a four ton dynamite blast was set off, marking the last „big shot” before construction of Hoover Dam proper begins near Las Vegas, Nevada. The explosives were detonated from 3,000 drill holes honeycombing the section of earth to be moved, a rock ledge on the lower canyon walls selected as the site for power houses. The roar of the explosion echoed through the walls of Black Canyon for 15 minutes. — Image by © Bettmann/CORBIS