Underground Marvels

A hidden, subterranean city built within the Rock of Gibraltar was an immense fortress to control the seas during World War II, and new discoveries reveal the secrets of a long-lost chamber with evidence of high-stakes espionage.

A strange, centuries-old labyrinth built into an abandoned quarry was built by an eccentric millionaire, and no one knows why; but now, a new excavation into this mysterious 9,000-foot tunnel system might finally reveal its dark secrets.

A vast system of tunnels built almost three thousand years ago was the site of the greatest battles of antiquity, and these disorienting passages could be the deadliest weapon in war; experts reveal what made this underground world so terrifying.

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.

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.