Popular Mechanics did this neat article describing some of the most popular US top-secret aircrafts that have been mistaken for UFOs.

According to PM, “The CIA estimates that over half of the UFOs reported from the ’50s through the ’60s were U-2 and SR-71 spy planes. At the time, the Air Force misled the public and the media to protect these Cold War programs; it’s possible the government’s responses to current sightings of classified craft—whether manned or remotely operated—are equally evasive. The result is an ongoing source of UFO reports and conspiracy theories.”

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Source: Popular Mechanics