The Nazis had plans to blow up the Hoover Dam during World War II, in an effort to cripple aircraft manufacturing in Los Angeles.
Born out of the Great Depression and completed in 1935, the dam was the largest ever built and stood as a symbol of America's ability to overcome adversity. It fueled Southern California's incredible growth – its large cities, its industrial base, its massive agricultural industry, and the nation's biggest defense plants, according to the National Archives.
This video from American Heroes Channel gives an idea of what happened:
Fortunately, the government was tipped off to the plot and upped security in the area. But it kept fears of the plot secret for more than 60 years, until a historian unearthed documents while doing research at the National Archives, according to Mental Floss.