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

A chain of 16 wondrous lakes formed as a result of the gradual dissolution of soluble layers of bedrock.

Roman ruins, natural methane fires and a lovely beach.

Most prolific publishing house in the 1600s open to the public.

A Moroccan getaway in the mountain range that plays the Himalayas' stunt double.

World's largest film studio, where visitors tour the decaying sets that litter the harsh desert environment.

Natural stone arches form a walkable "cathedral" at low tide, then disappear entirely when the water returns.

Commonly acknowledged as the only public monument to the Devil himself.

Mysterious Spanish catacombs of unknown origins.

An ancient Egyptian temple in the middle of Madrid, Spain.

Devastated landscape used for hundreds of years by the Romans as their primary mining site.

A small Portuguese cottage formed between boulders in the countryside.

A submerged village appears when dam levels drop.

Picturesque intersection of witchcraft and ancient Christianity.

Houses are tucked between, on, and underneath giant boulders.

Legacy of the earthquake that nearly wiped Lisbon off the face of the Earth.

A 16th century chapel decorated with bones, skulls, and entire bodies hanging from the wall.

The corpse bride of Portugal.

Pagan monument converted to Christianity with the population.

This iconic Parisian bookstore doubles as the "Tumbleweed Hotel" for traveling writers.

The tallest stationary dunes on Earth also happen to sing.

A stunning silver lining to global warming's gloomy cloud.

This "floating island" plateau sits 7,671 feet above the forest floor.

Hundreds of hidden passageways weave through the old quarters of Lyon.