A nearly unconquerable island-fort in the Indian Ocean.

A centuries-old lookout post still sits atop a natural stone spire.

An extraordinary "City of Caves" carved from solid stone.

The only Chinese temple in Mumbai.

An expansive complex of subterranean tunnels is all that remains of one of Europe's most formidable medieval fortresses.

A spectacular cave and a beguiling lake.

These ruins in Volvi were once at the intersection of the Eastern and Western Worlds.

A minotaur, a labyrinth, and a dubious restoration pull in visitors from around the world.

This field of half-buried stone crosses was the burial place of a sect of heretical gnostics.

Thousands of rusting train cars have been slowly falling apart on the edge of Greece's second largest city.

A two thousand year old brothel uncovered in Northern Greece.

This architecturally eclectic early-20th century mosque was built as a place of worship for the Islamic followers of a Jewish messiah.

This open air archeological site houses almost as much wildlife as it does classical religious artifacts.

Monasteries on rock pillars, once accessible only by frayed ropes.

This 15th-century island fortress later served as home to the executioners no one wanted to live near.

This legendary city has been a tourist attraction for centuries.

The hallucinogenic center of the ancient world.

The hidden lair of an alchemist rediscovered in a 2002 flood.

A Jenga-style cylindrical tower of books that appears to go on forever.

This Prague museum collects brutalist relics and memories from the country's former Soviet rule.

Dedicated to the unsung history of human waste.

The first museum in the world devoted to sexual gadgetry.

The last municipal pneumatic post system in the world was in operation for over 100 years, and may run again.

This artificial cave looks like something out of a fairytale.