In six classic programs, National Geographic explores great secrets of the past.

The fourth season of the television series analyzes alien evidence here on Earth to investigate extraterrestrial forces and Earth's natural disasters; extraterrestrial beings, the Mayans' disappearance and the doomsday prophecies; and the existence of grey aliens.

This season travels the universe across an expanse of tens of thousands of years to the present day as we investigate the possibilities and probabilities of life beyond Earth. We look at the unexplainable, enormous man-made caves, structures, crop circles containing encrypted messages, and complex mathematical and astronomical equations perplexing our leading scientists. Experts have found thousands of strange drawings of beings with blue skin, multi-limbed...

1,200 years before the birth of Christ, the Aegean was ringed with vibrant civilizations. A century later, the Egyptians alone retained a measure of their former strength. What happened in those hundred years has long been a source of mystery and fascination for scholars of the ancient world. Travel from the banks of the Nile to the mountaintop remains of Minoan fortresses on Crete in search of clues to what happened. Could a mysterious warrior nation...

"In the early 1970s, Nobel Prize-winning DNA co-discoverer Sir Francis Crick and his colleague Leslie Orgel proposed that in the distant past, an extraterrestrial race sent a spacecraft loaded with microorganisms to seed the Earth with life. Now, more than 40 years later, the fields of space research and biotechnology have advanced to the point where they can back up Crick and Orgel's claims about our ancient alien ancestors. Sharing scientific evidence...