Hoover Dam
The Boulder Dam was built during the Great Depression to tame the Colorado River and produce electric power. The concrete dam was built in under 5 years by over 5000 workers, and was turned over to the federal government in 1936. The dam was officially renamed to Hoover Dam in 1947.
The dam was constructed of 5 by 30 by 30 foot concrete blocks. Each block contained steel pipes through which refrigerated water was run to help the concrete cure. To give the concrete time to sufficiently cure, builders limited construction to 7 blocks high within a 30 day period.