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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.

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.

An eerie system of underground vaults in Scotland was once the hideout for thieves who stole newly-buried bodies and sold them; today, experts are excavating these rubble-filled caves to reveal the secrets of what these bandits left behind.

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.

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 world's biggest labyrinth is a 1,553-mile cave system that's home to some of history's darkest mysteries, and new discoveries at this strange place reveal the long-lost secrets of Nazi occupation, Soviet revolution, and Cold War subterfuge.

A labyrinth of tunnels in the French countryside has served as a secret hideout during wartime for centuries. On its walls, experts have discovered the final confessions of soldiers who fought here during one of the deadliest days of World War I.

A mysterious cave system is home to some of the oldest human remains ever discovered, and today, the explorers and researchers investigating this site are uncovering shocking evidence of cannibalism among its Ice Age denizens.

A strange underground system known as the Black Cave was home to an infamous nobleman on the run from the law; today, experts investigate the secrets he left here to discover new parts of this mysterious network.

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.

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

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.