It is one of the greatest mysteries of the prehistoric world: why did people on nearly every continent erect giant standing stones? According to Ancient Astronaut Theorists, the placement of these monoliths and their geological makeup could point to extraterrestrial intervention–and, quite possibly, a technology that once existed on Earth far more advanced than our own.

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The Top Ten Extraordinary Creatures

Ancient Aliens explores the controversial theory that extraterrestrials have visited Earth for millions of years. From the age of the dinosaurs to ancient Egypt, from early cave drawings to continued mass sightings in the US, each episode in this hit HISTORY series gives historic depth to the questions, speculations, provocative controversies, first-hand accounts and grounded theories surrounding this age-old debate. Did intelligent beings from outer space visit Earth thousands of years ago?