During World War II, Hoover Dam was considered critical to the war effort because of the electricity it generated and the water stored in Lake Mead. America’s enemies also recognized its significance, and as early as 1939 a plot by foreign agents living in Las Vegas to bomb the intake towers was uncovered and prevented. A few more minor incidents occurred during the war, but the dam was never seriously threatened. After the December 7, 1941, attack on Pearl Harbor, no private vehicles were allowed across the top of the dam without a military escort for the duration of the war. The hill above the dam held one of four camouflaged pillboxes armed with machine guns and manned around the clock. Gordon B. Kaufmann, who created the dam’s architectural design, suggested stringing steel cables and armor plating across Black Canyon to protect the dam from bombs, and Allen Tupper True, designer of the dam’s floor inlays, proposed camouflaging the dam and building a decoy dam downstream, but neither idea was implemented. The City of Henderson grew up around the Basic Magnesium Plant, which used one-quarter of the electricity generated at Hoover Dam during World War II, and the magnesium was used to build airplanes and incendiary devices.