In 1935, Hoover Dam (or Boulder Dam) was a brand-spanking-new feat of engineering, dedicated in a ceremony attended by Franklin D. Roosevelt in September of that year. Less than four years later, government officials feared it would all be demolished by Nazis.

A number of suspicious activities were reported beginning in October 1939, including a German man who took large numbers of photographs around the dam and was upset when his female companion accidentally strayed into some of the shots.

By November, the State Department had word from the U.S. embassy in Mexico that two German agents planned to bomb the dam’s intake towers and cut power to the high-voltage line, with the goal of crippling the aviation manufacturing industry in Los Angeles, which did indeed rely heavily on the hydroelectric power provided by the dam. The agents planned to rent a boat under the guise of a fishing excursion, and would then use the boat to plant the bombs at the intake towers. One of the German agents had reportedly already made more than a dozen planning trips to the dam.

Officials took the threat quite seriously, immediately halting all recreational activity on Lake Mead. The restrictions applied to employees as well, which the War Department believed to be the biggest threat. None of them were allowed to enter the dam except when absolutely necessary from an operations standpoint.

Even with the precautionary measures, strange activity continued in the area. Shots were fired at a National Park Service patrol boat, and an unauthorized car was spotted driving away from a no-trespassing zone near the switchyard.

Though the discovery of the Nazi plan wasn’t made public, people noticed the sudden restrictions. Rumors began to circulate; one popular theory pertained to a massive net stretched across the lake, just above the dam, to catch any explosive devices that might be thrown at the structure. To calm the public, Bureau of Reclamation Commissioner John Page issued a press release in January 1940 saying that “Boulder Dam is perfectly safe. There has been no ‘plot’ unearthed. Reports that the Bureau of Reclamation is fearful that someone will dynamite the dam are ridiculous.”

Meanwhile, the Bureau was actually trying to figure out more advanced ways to protect the dam than access restriction and extra patrol. One “color consultant” recommended painting the dam and spillways with “bold, simple masses of colors” to help conceal the dam from planes overhead. Another proposal included building a three-quarter size “dummy” dam downstream from the real one. The decoy would be made of wire, then painted various colors and textures to simulate the concrete and rocks of the cliffs.

Despite all of the behind-the-scenes plotting and planning to protect Hoover Dam, the government continued to keep the plot from going public—and, in fact, none of this was discovered until 60 years later. In 2001, a historian for the Bureau of Reclamation happened upon government documents while doing research in the National Archives. The previously classified information revealed what citizens had been told was a completely unfounded fear: The Nazis had planned to blow up Hoover Dam.