This impressive chasm is one of the longest natural underground cavities in the world.

Explore an eerie, abandoned medieval citadel.

Green labyrinth formed by a karstic maze hidden in a preserved oak forest.

Tables of secondhand books fill the inner courtyard of this 17th-century stock exchange.

Beautiful carved tunnels.

A public park off the beaten path with a beautiful waterfall and 13th century Cistercian abbey.

Ivy-covered Medieval tunnels cut through this charming town in France's Loire Valley.

Historic medieval fortified city in southwest France.

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

A cave dwelling that was started by the Celts, expanded by monks, and finished by knights.

Stroll through retro Paris while shopping for old postcards and discarded doll heads.

Sacked by Vikings, blessed by William the Conqueror, dealt a fatal blow during the French Revolution—these ruins have seen it all.

This French ghost town from World War II has been left as a memorial to German atrocities during World War II.

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

Down a cobblestone alleyway is a museum dedicated to Romanticism, with a secret garden.

France's national museum of scientific and industrial instruments.

A museum of magic with a Sadist past.

An "inside out" building in the historical center of Paris, featuring Europe’s largest modern art museum.

A large instrument played by the wind.

Beyond being incredibly fun, not much can be said about the Hole with certitude.

A Roman-style column in Oregon hides a spiral staircase inside.

Graffiti, cars, and religion combine to create this jungle of scrap in the Nevada desert.

Preserved Inca city and site of the last Inca stronghold against the Spanish conquistadors.

The stones of this Incan wall complex fit together so well you couldn't fit a piece of paper between some of them.