I recently stumbled upon a CIA document online (through Defence Tech) that sheds light on the supposedly existence of unidentified flying objects (UFOs). The document claims that the sightings, especially in the period 1947-60s, can be attributed to flights of secret aircraft, such as U2 and SR-71 spy planes, and the CIA and US Air Force made deceptive and misleading statements about them to protect the existence of such. More than half of all UFO sighting can be traced to such flights.
Defencetech.org is also reporting that early U2 planes were painted with radioactive coatings (that can minimize its radar signatures). The coatings gave the aircraft a visible glow at night.