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 3,000 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 is connected to a fairy-tale castle and 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.

Beneath Naples in Italy, the ancient Greeks excavated a 280-mile network of tunnels 130 feet underground. Scotland's capital sits atop a network of tunnels that became a den of criminal activity, but have now been reimagined for the 21st century.

Beneath Dover Castle lies an extraordinary underground world created to thwart Hitler in the biggest rescue operation of WWII. In the Netherlands, an 18th-century fort sits above the St. Pietersburg Caves, a limestone system that runs 155 miles.

Beneath the city of Istanbul, there are reportedly a staggering 200 ancient subterranean cisterns, with more still being discovered. In France, the Maginot Line was a vast network of underground fortresses designed to stop a Nazi invasion.

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

A buried ancient city is 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.