Investigators circle the globe in search of evidence in their quest to determine whether life on Earth began in outer space and if aliens influenced mankind in ancient times. Did extraterrestrial beings visit Earth and share information about technology and influence human religions? And more importantly, if aliens visited the planet before, will they return? Alien theorists believe that the answer to both questions is a resounding yes.
UFO Hotspots: Studies show 70% of UFO sightings occur in 'hotspots', where unusual phenomena is regularly reported. Is there something about these places that attracts visitors? (S15, ep 1)
More than 10,000 crop circle formations have been documented and many display properties that defy explanation. Could they be of otherworldly origin? (S15, ep 2)
Mystery of the Lost Civilisation: The discovery of the ancient Karahan Tepe megalithic site, displaying perplexing engineering skills, turns the archaeology world on its head. (S15, ep 3)
Scholars identify Egypt's obelisks as monuments to the pharaohs, while ancient astronaut theorists suggest they may power communication with the gods. (S15, ep 4)
The Mutual UFO Network is devoted to the scientific investigation of UFOs. With more than five decades of investigations, MUFON's database could contain proof. (S15, ep 5)
Meteor impacts have created the perfect environment for humankind. Could new evidence reveal that celestial objects have been sent by an alien intelligence? (S15, ep 6)
Explore whether contact with extraterrestrial life could have occurred. If so, has this happened in the highest levels of the US government? (S15, ep 7)
The vast Alaskan wilderness is known for disappearances, stories of strange creatures and UFO sightings. Could this be due to an otherworldly presence? (S15, ep 8)
Eerie encounters at 40,000 feet, accounts of UFOs taking control of aircraft, and jumbo jets disappearing. Is this proof of aliens in our airspace? (S15, ep 9)
Could the megalithic structures of Malta - and an underground complex housing otherworldly remains - be linked to local legends of alien visitation. (S15, ep 10)
Count down the world's top ten pyramid sites, from hidden structures in Peru to the famous landmarks of Giza in Egypt. (S15, ep 11)
Count down the top 10 mysterious devices found around the world, from strange discs found in a cave in China to an ancient Greek computer. (S15, ep 12)
Count down the world's top 10 mysterious islands, including sites said to be home to giants and islands that disappear into thin air. (S15, ep 13)
Top Ten Alien Craft: Count down the top ten alien craft, from V-shaped objects to orbs of light to flying saucers. (S15, ep 14)
Edgar Cayce - The Sleeping Prophet: In the 1900s, a modern-day prophet Edgar Cayce became a worldwide sensation. He claimed to receive visions of both the past and future. (S15, ep 16)
Explore whether the Gods of Greece including Zeus, Apollo and Poseidon, were actually extra-terrestrial visitors to Earth. (S15, ep 18)
In the 21st century, UFOs have gone from fringe science to mainstream obsession. Are we getting closer to understanding the strange events in the sky? (S15, ep 19)
For millennia, people have believed that everyday objects can possess hidden energy. These talismans are seen to provide protection and good fortune. (S15, ep 20)
Count down the top ten mysteries of the deep, from sunken continents to supposed proof of the lost city of Atlantis. (S15, ep 17)
Count down the top ten alien petroglyphs around the world, from strange figures to flying craft and carvings of what appear to be astronauts. (S15, ep 15)