If ancient aliens visited Earth, what was their legacy, and did they leave behind clues that exist in plain sight such as sophisticated aircraft, complex electrical grids, and intricate construction machinery? Indian Sanskrit texts, dating back to 6000 B.C., describe in varying but vivid detail flying machines called Vimanas. Megalithic stone structures in Egypt reveal evidence of precision saw work. Interpretations of the Jewish Zohar writings offer depictions of a life-sustaining manna machine, eerily similar to chlorella algae processing systems today. Are these examples of modern technology, or is there evidence that these incredible mechanisms existed on Earth thousands of years ago?

If ancient aliens visited Earth, who were they, and where did they come from? Possible historic evidence and beliefs are examined around the world. The Dogon people possess knowledge of a galaxy they claim was given to them by a star god named Amma. The Hopi and Zuni people celebrate Kachinas, gods from the sky, whose headdresses and costumes appear to resemble modern helmets and protective clothing. Halfway around the world, Chinese legends tell of the Han leader, Huangdi, arriving on Earth on a flying, yellow dragon. Was this dragon more likely a spacecraft? Ancient astronaut theorists believe that these are far from chance encounters and that extraterrestrials not only interacted with us, but changed the course of human history.

If ancient aliens visited Earth, what was their mission, and is there evidence that points to when they will return? Ancient Sumerian tablets describe an alien race, known as the Anunnaki, who came to Earth to mine for gold. Egyptian hieroglyphs depict hybrid creatures that were part man, part animal. Mysterious crop circles and unexplainable crystal skulls are thought to contain messages that man has yet to decipher. Strange evidence may exist in multiple cultures throughout history suggesting that aliens came here to explore, excavate raw materials, experiment on human beings, and even for pure conquest.

Reports of encounters with strange beings and sightings of mysterious objects in the sky have occurred throughout history. A 13th century historical book, Otia Imperialia, includes an account of a creature descending from a flying craft over Bristol, England. The log from Christopher Columbus' first voyage to America contains a report of strange lights in the sky. Medieval art pieces depict disc-shaped objects floating in the heavens. Sightings of flying cigar-shaped crafts were reported during the Black Plague. And there were even discussions of extraterrestrial life among America's Founding Fathers. Could these sightings, coming from every part of the world, from biblical times to present day, be evidence that aliens have been with us all along?

There is evidence that suggests we experienced 20th century alien contact. In 1942, the Battle of Los Angeles involved the US military and Air Defense allegedly fighting a UFO. WWII Allied Air Force planes were buzzed by balls of light that some think could have been extraterrestrial. And the alleged crash of a flying saucer in Roswell, New Mexico in 1947 propelled the world into the age of UFOs. For decades, some people have been searching for the sounds of extraterrestrial intelligence, while others have been sending messages out into deep space. What would happen if aliens answered... or came calling? What protocols exist to determine who would speak on behalf of mankind, and how might we communicate? What happens if they return? Some believe they already have.

What if life on Earth began in outer space? Millions of people accept the theory that intelligent life forms visited Earth thousands of years ago and were worshiped as gods by primitive man.