Don explores top-secret military and government tunnels and looks into classified technology that could be used to build entire cities right under our feet. He tours a Titan II missile site near Green Valley, Arizona; roams outside Area 51 and Nellis Air Force Base, Nevada; explores a tunnel boring machine (TBM) at the Three Rivers Tunnel Project near Fort Wayne, Indiana; examines the potential for nuclear-powered tunnel boring; tours the former emergency relocation bunker for the United States Congress under West Virginia's Greenbrier Resort; and investigates U.S. Army underground urban warfare training in Indiana.

Don goes deep beneath the Great Lakes and explores flooded caves as he searches for evidence of a mysterious, cave-dwelling, pre-Clovis society that may have been the first people to reach America. Evidence presented: a mastodon image carved on a rock beneath Lake Michigan, 16,000-year-old poop and arrowheads from a dry cave in Central Oregon, and a 13,700-year-old human female skeleton from Cenote Naharon in Mexico's Yucatan. Nicknamed Eve of Naharon, she is the oldest human skeleton found in the Americas.