A series of strange white cross-like formations can be spotted above a section of Arizona desert, and an investigation has revealed that they were part of a secret project by the U.S. military to spy on enemy territory during the Cold War.

About 12 years ago, the pilot of a small plane spotted some unexpected formations while flying over a section of Arizona desert. 
As a recent NPR report about the sighting reveals, she and a friend investigated the cross-like structures and discovered that they “were once part of a top-secret government program called CORONA — the nation's first reconnaissance satellite program.” 
CORONA was, in fact, a part of the U.S. efforts to spy on the Soviet Union, China, and other Cold War threats during the 1960s. 
American officials decided to secretly launch satellites with cameras to take photos of enemy territory without being detected.
As the initial images were out of focus, the military installed around 270 50-foot-wide X markers.
According to Atlas Obscura, "The crosses were roughly arranged into a 16 x 16 grid so that the satellites could orient themselves and focus correctly before actually spying on America's Cold War interests."
The location was considered ideal because of its “extremely clear weather.” 
The project was successful and helped to produce 800,000-plus photos of enemy airfields and nuclear weapons sites. 
CORONA ended after about 12 years in 1972, and though some of the concrete markers have been destroyed, others remain as a reminder of the Cold War.