This modern French ossuary contains the jumbled bones of 130,000 WWI soldiers.

An aluminum skeleton brings Chinese mythology to the French shore.

A well-preserved medieval village nested on a rocky outcrop and surrounded by a network of cliffs, natural bridges and canyons.

A French valley with forests so lush it was nicknamed "the Small Amazonia of the Pyrenees."

Do these monsters look familiar? The gargoyles on this church are all from pop culture.

A Gothic cathedral created from volcanic rocks.

The largest underground church in Europe.

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

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.

Exquisite wall sculptures carved in secret by an early quarryman in an obscure corner of the Paris Catacombs.

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

The world's largest underwater restaurant is submerged off the southern tip of Norway.

The convergence of two currents makes this a surprisingly pleasant getaway in the Arctic Circle.

An underground nightclub helped spark a "Singing Revolution."

This stunning limestone waterfall is the highest in Estonia.

A massive bridge forms part of an aqueduct that was deemed an architectural wonder of its time.

The pristine waters crossing this forest form a hidden waterfall among the dense vegetation.

A magnificent waterfall is also home to the remains of a Roman hydraulic engineering system.

Built to be the ideal penitentiary, this abandoned panopticon saw several revolts and even a prisoners' coup.

A mushroom-shaped outcropping that has come to be the symbol for an entire town.

This serene, turquoise lake in an extraordinary nature reserve showcases a rare natural phenomenon.

Italy's curious field of mud volcanoes is named for being both gross AND salty.

Take a giant yellow bird cage to the top of Elba's highest mountain.