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 hills of Kentucky, and containing prehistoric artifacts and ancient human remains, experts reveal a subterranean mystery that may have changed the course of U.S. history.

Buried nearly a mile beneath the Black Hills of South Dakota is a cutting-edge laboratory at the forefront of astonishing scientific research; 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.

Experts investigate a decommissioned underground missile launch facility.

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.

Experts explore an extensive network of tunnels beneath Ramsgate, England.

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