This castle-fortress is a living example of the transition from medieval defenses to modern fortifications.

The world's largest solar furnace uses 10,000 mirrors to concentrate the sun's rays to generate power.

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

This tiny hotel perched atop a 50-foot tower really puts things into perspective.

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

French commune at the heart of conspiracies.

France's high-tech, future-themed amusement park.

These medieval ruins have survived since the 12th century, even providing shelter from the Nazis.

Series of underground houses inhabited continuously for centuries.

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

This narrow path was cut right into a French mountainside to feed the country's early naval expansion.

This submersible road connecting a French island to the mainland is lined with safety towers for when the tide strands travelers.

Bits of architecture from a town destroyed in World War I now decorate this hotel garden.

This futuristic observatory at the top of the Pyrénées offers unrivaled views.

A centuries-old complex of hand-dug stone chambers has sheltered locals in times of trouble.

One of the largest ochre quarries in the world, hidden in a limestone landscape.

The deserted pirate-themed restaurant still entices explorers.

The post-apocalyptic remnants of a bucolic town ruined by a plane crash.

Hundreds of dwarf beeches create an unusual, haunting forest in the north of France.

An isolated 17th-century chapel on a remarkable site once dedicated to Celtic sun worship.

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

A bike train runs along a disused railway line through the tranquil French countryside.

The 16th-century church was kept in its ruined state as a reminder of the casualties of World War I.

An abandoned beach resort in Normandy reclaimed by artists.