This whimsical castle of grottoes was the labor of love of a French postman, and one of the greatest achievements of outsider architecture.

Tubing through a cave lit with glowworms is a magical underground experience.

Walk 2,300 feet above the ground on the "Troll's Tongue."

Millions of glowing microscopic organisms light up the waters of this luminous lagoon.

The ghost town left by the worst nuclear disaster of all time is being taken over by nature and urban explorers.

A mysterious ancient cemetery with Roman sarcophagi and Lycian inscriptions.

A Lycian city of the dead carved into the hillside of Southern Turkey, complete with startlingly human touches.

A Turkish ghost town abandoned in a population exchange with Greece.

A gargantuan crystal kingdom.

Natural catch-basin rocks covered in thousands of drawings spanning many millennia.

Explore the largest pure gypsum deposit in the world, and go dune sledding while you're at it.

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

A muddy alien landscape that is home to rare species of exotic plants.

The remains of diamond fever taken over by the desert.

A part of the world's largest marsh seems to be growing bright red Martian grass.

Thousands of exquisitely carved stone water storage wells, hundreds of stone steps fill this exquisite medieval stepwell.

In this historic region, homes and churches are carved into ancient volcanic rock formations.

Cliffside houses provide a spectacular view from their perch above the valley.

Naturally protected city carved from a rock face.

One of the few remaining ancient aqueducts described by Frontinus as 'the most solemn testimony of the Empire.'

Keshwa chaca, among the last handwoven Incan bridges, crosses Apurimac Canyon in Peru.

These quaint French houses dangle precariously over a precipice.

Abandoned medieval hill town.

The world's most active stratovolcano is almost always erupting.