Cities of the Underworld - Season 4 (2021)

Don goes deep beneath the Great Lakes and explores flooded caves as he searches for evidence of a mysterious, cave-dwelling, before the 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.

Don explores newly discovered ruins and caves in Croatia as he uncovers the real story of a formidable queen who dared to defy the mighty Roman Empire.

Don goes deep beneath Bagras Castle, a Knights Templar stronghold in Turkey, to explore a provocative theory that may link the fabled secret society to Freemasonry and America's founding fathers. He also explores the Old North Church in Boston, Massachusetts and Fort Mifflin in Pennsylvania.

Don explores the massive underground cities of Cappadocia in the mountains of Turkey that may have been home to the first civilization on Earth. Also explored are Göbekli Tepe.

Don follows the clues hidden underground as he searches for evidence of 10,000 Aztecs who fled Mexico and disappeared somewhere in America.