A real-life "hobbit" cave in Flores, Indonesia.

It's safe—and permitted—to walk across these twin natural sandstone arches.

These wondrously illuminated caverns extend under the mountains of the Fukushima Prefecture.

A series of caves, tunnels, and wells filled with fascinating figures and symbols.

The deadliest diving spot in the world is also a stunning geological marvel.

This coffee shop lies hidden beneath a historic church.

While not Turkey's deepest underground city, this subterranean redoubt is the largest.

The deepest underground city in Turkey was designed to protect 20,000 people behind massive stone doors.

The secret room where early Bolsheviks cranked out propaganda fliers on a smuggled printing press.

Descend 2,030 years into the past in an ancient tunnel built to support Jerusalem's most famous wall.

Inside this mountain lies several kilometers of breathtaking caves.

This cave system allegedly holds the "metallic library" of a lost ancient civilization.

The world's second-largest excavated hole.

Ancient rock-hewn town that witnessed key events of Georgian history.

Hideout of thieves, monks, and goat-footed gods.

This complex of vertically-excavated Buddhist, Jain, and Hindu monolithic temples in India's Maharashtra providence illustrates a spirit of tolerance characteristic of ancient India.

An ancient Ethiopian cave church is endangered by the simple building of a road.

Once the most densely populated place in the world, this island is now a ghost town.

The natural hot spring that was integral to the creation of Canada's first national park.

This building, previously used as a courthouse, has an unusual geological feature in its basement.

This hole formed by a glacier is the largest and best-preserved of its kind in Stockholm.

Some of the freshest vegetables in Seoul are grown in a subway station.

Spectacular mineral formations fill these caves, which have been used for everything from smoking meat to stashing illicit liquor.

A frozen oasis lies beneath the scorching New Mexico desert.