Black and white image of the construction phase of the Boulder Canyon Project, also known as Hoover Dam. This image features the first round of drilling to enlarge the upper portal of diversion tunnel No. 2. The welded steel "Jumbo Rig" mounted on the truck has 25 air drills on it. The swing shift crew standing next to the rig prepare for work. Note: Boulder Dam was officially renamed Hoover Dam in 1947.