In England, the mysterious monument of Stonehenge stands as an astonishing feat of ancient engineering—yet how it was built remains one of history’s greatest enigmas. See more in this clip from Season 21, Episode 15, "Britain's UFO Files." New Episodes Thursdays at 8/7c Watch full episodes and stay up to date on all of your favorite shows 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 Instagram - https://www.instagram.com/history TikTok - https://www.tiktok.com/@history "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.