Six Companies' 1936 concrete arch-gravity dam on the Colorado River — Depression-era technological landmark.
From Wikipedia
The Hoover Dam is a concrete arch-gravity dam in the Black Canyon of the Colorado River, on the boundary between the U.S. states of Nevada and Arizona. Constructed between 1931 and 1936, during the Great Depression, it was dedicated on September 30, 1935, by President Franklin D. Roosevelt. Its construction was the result of a massive effort involving thousands of workers, and cost over 100 lives. Bills passed by Congress during its construction referred to it as Hoover Dam, but the Roosevelt administration named it Boulder Dam. In 1947, Congress restored the name Hoover Dam.