Keystone via Getty Images, file
1928: An inspection party near the proposed site of the Hoover Dam (previously known as the Boulder Dam) in the Black Canyon of the Colorado River.
Alfred Eisenstaedt / Time & Life Pictures via Getty I
1938: Sightseers enjoy the magnificent power of Boulder Dam, later renamed the Hoover Dam.
Alan Band / Keystone via Getty Images file
1972: The Hoover Dam near Las Vegas, Nevada on the Colorado River lit up at night.
I wonder if the men in the first image could have possibly imagined what the dam would look like, how many people it would take to build it and how many would lose their lives while it was under construction.
The 726.4-high concrete expanse on the Colorado River was dedicated on Sept. 30, 1935, by Franklin D. Roosevelt. Formerly named for Republican president Herbert Hoover, it was christened as less political Boulder Dam by Democrat Roosevelt, then later reverted to its original name. More than 100 workers were killed during the dam's construction.
More about the Hoover dam on wikipedia.
Events around the anniversary from the Las Vegas Review-Journal.