The former global standard for the exact mass of one kilogram sits under lock and key outside Paris.

This striking fort on the Maginot Line looks just as it did after being destroyed in the Fall of France.

The world's largest boar is a huge steel titan found at a French rest stop.

The haunting concrete remains of a Nazi pebble crusher can still be found in this small French commune.

This small red school was where Germany signed their surrender during WWII.

A massive, secret marina for the Nazis' most formidable weapon in the Atlantic.

A memorial to the nearly 10,000 American soldiers laid to rest on the beaches of Normandy during WWII.

Legend has it this 11th-century castle was paid a visit from the devil.

Series of underground houses inhabited continuously for centuries.

This modern museum of Mediterranean culture is surrounded by a surreal concrete honeycomb.

Miles of man-made caves and tunnels are currently used for cultivating mushrooms.

Underneath a chalk hill in France lies what Hitler hoped would be the underground launching base for a devastating new weapon.

A swath of France so devastated by war it is still forbidden to go there.

The Church of the Annunciation was carved into the face of a cliff, just like the rest of the town of Haute-Isle.

Original cabaret handbills, posters, and feline artwork are a purrfect highlight of the Museum of Montmartre.

There has been an astronomical clock on this spot since the 14th century.

This French castle is said to have once held the Holy Grail but is now a gorgeous mountaintop ruin.

This stunning French estate was so lavish that it landed its original owner in jail for the rest of his life.

The Cimetière des Chiens et Autres Animaux Domestiques is one of the oldest pet cemeteries in the world.

This stunning natural double arch, the largest in the Alps, went undiscovered until 2005.

Beautiful carved tunnels.

The fantastic architectural models of a French traveler.

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

The mental institution where Van Gogh created some of his most famous work now allows guests to walk in the artist's footsteps.