Underground Marvels - Season 2

The top-secret construction of a massive nuclear bunker remained concealed under a palatial West Virginia hotel for decades, and now, experts reveal how this vast subterranean structure was one of the most closely guarded Cold War secrets in US history.

The longest known cave system in the world spans hundreds of miles beneath the wooded hills of Kentucky, and containing prehistoric artifacts and ancient human remains, experts reveal a subterranean mystery that may have changed the course of US history.

Deep beneath the streets of Cincinnati lies a network of deserted tunnels and caverns that contain a remarkable history of the city's origins, and now, experts investigate to reveal who built this underground labyrinth and why it was abandoned.

Buried nearly a mile beneath the Black Hills of South Dakota is a cutting-edge laboratory at the forefront of astonishing scientific research, and now, experts reveal advancements engineers achieved more than a century ago that keep it operating today.

Subterranean military fortresses situated on the southern coast of Ireland have protected against enemy invasion for over 400 years, and now, experts investigate how this powerful stronghold transformed into a brutal and terrifying prison.

A decommissioned missile launch facility in Kansas remained one of the best kept subterranean military secrets of the Cold War, and now, experts reveal how this abandoned underground complex transformed into a state-of-the-art survival bunker.

An unprecedented landslide unveils a hidden world beneath a picturesque town in northern Italy, and now experts investigate this vast network of tunnels and passageways that stretch 300 feet underground to reveal the city's subterranean past.

Descending more than 200 feet beneath the Arizona bedrock lies the largest dry cave system in North America; experts explore this vast subterranean network to reveal the mysteries and undiscovered treasure that remain hidden within its walls.

Hidden beneath the town of Ramsgate, England, lies a hidden network of over three miles of air raid tunnels; experts investigate and uncover an extensive WWII bomb shelter that became a permanent refuge for many civilians after a devastating attack.

A forgotten world of ancient underground passages and medieval engineering lies hidden beneath Exeter, England; experts investigate and reveal how this ambitious tunnel system resulted from one of the deadliest epidemics in history.