Throughout the world there are mysterious ancient stones shaped into perfect spheres. Many of them show clear evidence of being manmade and some completely defy explanation. Could the mystery of the stone spheres provide clues about humankind's alien past? See more in this scene from Season 20, Episode 10, "Mystery of the Stone Spheres."

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"Ancient Aliens" explores the controversial theory that extraterrestrials have visited Earth for millions of years.

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