This massive subterranean fissure is known as the "underground Grand Canyon."

An underground city of ancient quarry tunnels hid thousands of Allied soldiers in WWI.

One of the most remarkable caves in all of Iceland was created by a once raging river of lava.

This eccentric cave dwelling is bizarre even by the standards of a subterranean town.

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

One of the largest caverns in the world has a maw large enough to stack cathedrals in.

The hidden "ballroom" under a mansion lake was built by a criminal who ate a cyanide pill during his trial.

These numerous Israeli caves were dug out over hundreds years for reasons ranging from hideouts to burial sites.

This network of conjoined cellars was riddled with traps and supposedly allowed the townspeople to pretend their homes were deserted.

Enter the world under the hill at this ancient UK burial chamber.

Hide from the tourist hordes in these relatively unknown, and unguarded lava caves.

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

This volcanically volatile island town is home to an abandoned series of tunnels left by the Japanese during WWII.

Forgotten German bunkers in China create a sharp cultural contrast.

An exquisite tunnel of cascading flowers.

Legend says that this short Canadian tunnel still holds the dying screams of a girl who burned alive.

Known for the tunnel that leads visitors to the water.

This Irish show cave is said to have been the site of a Viking massacre that claimed 1,000 lives.

A rocky Irish outcropping with a history of wealth, conflict, and ruin.

A phenomenal assembly of pueblos in New Mexico is the most complete example of ancient ruins north of the border.

A desert that rolls right up to the ocean, where it's so dry animals live off the fog.

An enormous dragon scales the walls of this 17-story temple.

This fantastical waterfall is hidden in a cave located in a cracked cliff.

Hidden deep in the Guatemalan jungle is a turquoise paradise of natural pools, caves, and waterfalls.