The Freedom of Information Act: In 1969, the US government officially announced it would stop investigating UFOs, but declassified documents seem to tell another story. (S4, ep 3)

Astronaut Encounters: To this day, NASA denies all UFO accounts and has taken measures to prevent its astronauts from telling their stories. But what are they hiding? (S2, ep 3)

The 1952 Wave: In Spring 1952, reports of UFO sightings shot through the roof. Can former US Air Officials shed any light on this? (S4, ep 2)

Human Harvest: A look at frightening reports of alleged alien abduction, including cases in which abductees claim to have been subjected to torturous medical experiments. (S2, ep 2)

Second Skin: To enter space, humans require a space suit. Are the aliens that are said to have visited earth any different? Or are they more akin to artificial drones? (S4, ep 1)

Alien Origins: The compelling series returns to examine more eerie reports of UFOs. Can it determine where in the universe such strange flying crafts could be coming from? (S2, ep 1)

Stress: Psychologists take a closer look at the stress of flying, exploring the emotions affecting staff, pilots and passengers at airports and up in the sky. (S1, ep 8)

Mechanical Failure: Phone footage reveals what happens when mechanics fail at high altitude. It's aviation's worst nightmare, but what can the industry do to prevent it? (S1, ep 7)

Fires: Shocking phone footage reveals what it's like for a flight to go up in flames, and experts reveal how firefighters deal with aviation infernos. (S1, ep 6)

Weather: Science and passenger phone footage reveals what happens when planes get hit by typhoon winds, giant hail stones and lightning. (S1, ep 5)