A hidden, subterranean city built within the Rock of Gibraltar was an immense fortress to control th...

A strange, centuries-old labyrinth built into an abandoned quarry was built by an eccentric milliona...

A vast system of tunnels built almost three thousand years ago was the site of the greatest battles...

A strange underground system known as the Black Cave was home to an infamous nobleman on the run fro...

An eerie system of underground vaults in Scotland was once the hideout for thieves who stole newly-b...

The world's biggest labyrinth is a 1,553-mile cave system that's home to some of history's darkest m...

Caverns hidden deep inside a remote mountain are home to a precious mineral found nowhere else on ea...

A labyrinth of tunnels in the French countryside has served as a secret hideout during wartime for c...

A buried ancient city hidden five stories underground, and to keep its location secret from invaders...

A mysterious cave system is home to some of the oldest human remains ever discovered, and today, the...

The top-secret construction of a massive nuclear bunker remained concealed under a palatial West Vir...

The longest known cave system in the world spans hundreds of miles beneath the hills of Kentucky, an...

Deep beneath the streets of Cincinnati lies a network of deserted tunnels and caverns that contain a...

Buried nearly a mile beneath the Black Hills of South Dakota is a cutting-edge laboratory at the for...

Subterranean military fortresses situated on the southern coast of Ireland have protected against en...

Experts investigate a decommissioned underground missile launch facility.

An unprecedented landslide unveils a hidden world beneath a picturesque town in northern Italy, and...

Descending more than 200 feet beneath the Arizona bedrock lies the largest dry cave system in North...

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

A forgotten world of ancient underground passages and medieval engineering lies hidden beneath the t...