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UFO sightings over nuclear facilities trigger high alert responses - are they top-secret aircraft or something from another world?

An investigation into triangular-shaped UFOs seen in the 1980s and 90s - they might be the best evidence of extraterrestrial crafts?

Anti-aircraft guns blast the skies over Los Angeles when an unknown flying object is spotted by thousands in 1942 - is it from earth or could it be extra-terrestrial?

Roswell is the most famous UFO story in history and a piece of debris found at the desert crash site might shed new light on what really happened in 1947.

A look back at encounters between unidentified flying objects and airliners, including Japan Airlines flight 1628 in 1986 and Captain Ray Bowyer’s 2007 Channel Islands sighting.

Why do governments keep UFO cases classified? The episode follows Milton Torres, a USAF fighter pilot ordered in 1957 to fire on a radar target over Britain that then escaped.