Angkor Wat, Cambodia, is the world's largest ancient religious temple. Within its megalithic ruins, researches have discovered a depiction of a species of dinosaur--a stegosaurus. But how could the ancients have had knowledge of animals that mainstream science says died out millions of years ago? Why are dinosaurs extinct? Did an alien race of beings kill off the dinosaurs to make room for humankind?

Is it possible that sightings of alien beings or UFOs may actually be evidence of time travelers from the future? And might ancient astronauts actually be time travelers from our future? Scientists explore the theoretical possibility of time travel. How can it be achieved? And what might people from 10,000 years in the future look like?

Might an examination of Leonardo Da Vinci's masterful paintings, highly technical hand-drawn sketches, and private journals reveal knowledge of otherworldly technology and extraterrestrial beings?

People on every continent have claimed to see the footprints of Bigfoot. And ancient myths and legends speak of an ape-like giant stalking the earth. What if, as Ancient Astronaut theorists believe, there is evidence to connect Bigfoot with an alien species that once visited Earth in the distant past?

At nearly 14,000 years old, the ruins of Puma Punku are the oldest and most baffling on the face of the Earth. No one knows who designed and built this complex of sophisticated inter-locking blocks, and then vanished. Researchers investigate the ruins on-location in Peru and present new computer analyses. Forensic evidence on the ground, together with local myths and legends, suggest this site may have been designed and even once inhabited by a species of extraterrestrials.

NASA scientists, former astronauts and secret government files all point to the conclusion that there has been contact with extraterrestrial life in the past. Researchers examine this and other recent scientific evidence that may provide proof of alien contact in both the recent and ancient past.

There are numerous historical, religious and mythic accounts of ancient civilizations being wiped out by volcanoes, earthquakes, floods, and meteors. Did god, or nature bring about such disasters? Or is it possible that celestial beings had a hand in the ancient world's catastrophic disasters?

Who are the beings with big heads and giant eyes that seem to have been visiting our planet for millennia? Known by researchers as the "greys," these mysterious beings have been depicted in rock-art and carved stone figures throughout the ages and all over the world. Who are they, what planet do they come from, and what do they want?

The Maya created the most sophisticated calendar systems in the ancient world, and according to many scholars their Long Count Calendar will come to an end on December 21, 2012. What does this mean for mankind? Some researchers believe it will usher in the return of Bolon Yokte--a mysterious god associated with creation and war. Could this god actually be an extraterrestrial as some ancient astronaut theorists believe? Other scholars believe the end of the calendar will bring about a series of catastrophic events that may threaten the very existence of mankind. They point to a rare galactic alignment that will coincide with the end of the Mayan calendar as proof of an impending geological catastrophe. If these doomsday prophecies prove to be true, will it mean the end of the world as we know it? Or could it reveal some profound truth about the origins of the human race?

The Mayan civilization dominated Central America for nearly 2000 years, but by the 9th century A.D., the great Mayan cities were abandoned, and the Mayan people vanished. What could have happened to this advanced culture? Might the Maya have left clues as to how they surpassed other cultures in science, mathematics, time keeping, and most importantly, astronomy? In recent years, hieroglyphs have been decoded to reveal that the Maya were able to accurately predict shifts in the Earth's axis every 26,000 years. Inscriptions on the sarcophagus of their most prominent ruler, King Pakal, tell of a journey to the Milky Way. And a sophisticated calendar foretells of the end of the world in 2012. Most striking is their belief that knowledge was passed to them by otherworldly beings who descended from the sky. Could the ancient Maya have actually conspired with extraterrestrial visitors to plan the future--and even the very end of the world?