Sitting right out in the open despite their clandestine origins, the Corona Satellite Calibration Targets that dot the Arizona desert are not hidden, but their true purpose was not revealed for years after their installation.
The Corona spy satellite system was launched throughout the 1960s as part of a program designed for the United States to peer on the Soviet Union, China, and any other enemy or ally that might come under suspicion. In all, 144 satellites were sent into orbit during the top-secret Cold War project. After taking their pictures, the orbiters would jettison their film, which would float back to Earth and be collected by a plane in midair (a system that led to a number of crashed canisters).
Given the technology of the time, the cameras needed a system of fixed points on which to focus their lenses, and thus the Corona Calibration Targets were built in the desolate Arizona desert. Little more than large concrete crosses poured to be flush with the ground, the targets were only visible if one walked up to them or passed over them from a great height, like space. 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 US Army Map Service purchased the land for the crosses as office space before installing the extremely precise markers.
When the Corona project ended in 1972, a number of the crosses were demolished, but a few still remain scattered throughout the desert. Now the calibration points simply act as so many Cold War holdovers reminding people of an especially paranoid chapter in American history.