Ancient Aliens

The Mayan civilization dominated Central America for nearly 2,000 years, but by the 9th century, the great Mayan cities were abandoned, and the Mayan people vanished. What could have happened to this advanced culture?

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.

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.

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?

NASA scientists, former astronauts and secret government files all point to the conclusion that there has been contact with extraterrestrial life in the 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.

People on every continent have claimed to see Bigfoot's footprints. 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?

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?

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.

Angkor Wat, Cambodia, is the world's largest ancient religious temple. Within its megalithic ruins, researchers have discovered a depiction of a species of dinosaur -- a stegosaurus.