Could these underwater mysteries provide a record of extraterrestrial visitation? Find out in this clip from Season 19, Episode 19, The Top Ten Mysteries of the Deep.

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"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.

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