A new subterranean realm is discovered deep in the caves of Vietnam, in this clip from Season 18, "Secrets of Inner Earth." 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 ...

During an Arctic expedition, naval officer Richard Burn makes a very unusual discovery, in this clip from Season 18, "Secrets of Inner Earth." Watch all new episodes of Ancient Aliens, Fridays at 9/8c, and next day on The HISTORY Channel website at http://history.com/schedule. #AncientAliens ...