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.

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.

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.

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.

Caverns hidden deep inside a remote mountain are home to a precious mineral found nowhere else on earth, and experts and treasure hunters use cutting-edge tech to follow rumors of a mysterious long-lost mine in search of a payday.

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 buried ancient city hidden five stories underground, and to keep its location secret from invaders, its residents devised an ingenious trick; experts reveal what it was, and using the latest tech, they reveal the truth behind this strange place.

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.