When classified planes were being built in Area 51, the Soviet Union's spy satellites made out their shapes the same way people can tell if a car has been parked in a certain space on a hot day.
Regardless of what else has been housed at Area 51, the OXCART project found a home there in the late 1950s. OXCART's team was building a spy plane that would succeed the U2. During these tests, scientists rolled prototype planes along stretches of asphalt and checked to see if they could duck radar. Those that showed up on radar were scrapped, and the designs that went by unseen were refined.
Unfortunately, the lack of tree cover made OXCART easy pickings for Soviet surveillance. Soviet planes and satellites went over Area 51 regularly. American intelligence determined the Soviet's surveillance schedule, so they built small sheds that they could quickly roll the planes into. This process of rushing the prototypes under cover was known as "hoot and scoot."
The USSR did get an outline of an OXCART prototype, but not because the hoot-and-scooters were caught off-guard. See, the OXCART planes sat on the baking asphalt all day, blocking the rays of the sun. As the pavement around them heated, it gave off infrared light. When the satellites passed overhead, the area where the planes had blocked the sunlight stood out as a dark hole in a glowing surface. It was even possible to detect where the engines were on the plane.
Americans got word of the success of infrared scans and responded by making model planes. These sat on the runway, blocking the light and creating decoy dark patches. Sometimes, when they really went all out, the researchers put heaters under the decoys to make it look like the plane had just landed and heated the ground under it. It seems as though this strategy worked. The Archangel-12 came as a surprise (as far as anyone can tell) to everyone except those had built it. Score one for set dressing.
[Via National Geographic. Photo via Roadrunners Internationale via Pangloss Films.]