Ancient Astronaut Theorists believe that past evidence of ancient civilizations may have been destroyed. See more in this clip from Season 15, "The World Before Time." Watch all new episodes of Ancient Aliens, Fridays at 8/7c, and next day on The HISTORY Channel website at http://history.com/schedule. #AncientAliens Subscribe for more from Ancient Aliens and other great The HISTORY Channel shows: http://histv.co/SubscribeHistoryYT Find out more about the show and watch full episodes on our site: https://histv.co/AncientAliens Check out exclusive The HISTORY Channel content: History Newsletter - https://histv.co/newsletter Website - https://histv.co/History Facebook - https://histv.co/Facebook Twitter - https://histv.co/Twitter "Ancient Aliens" explores the controversial theory that extraterrestrials have visited Earth for millions of years. HISTORY® is the leading destination for award-winning original series and specials that connect viewers with history in an informative, immersive, and entertaining manner across all platforms. The network’s all-original programming slate features a roster of hit series, premium documentaries, and scripted event programming.