Once the world's largest reinforced concrete structure, this abandoned bridge now beckons urban explorers.

A modern-day Lascaux cave underneath Paris.

Abandoned railway line circling the city of Paris.

100-windowed castle turned art gallery, built by one dedicated mason.

Jean Tinguely's bizarre monster sculpture is hidden deep in a forest.

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

An enchanting work of outsider art, full of whimsical sculptures and carvings.

Series of underground houses inhabited continuously for centuries.

Dedicated to the history and culture of smoking, this small museum now includes a vape lounge.

Perhaps the most beautiful of preserved saints, with a little help from Paris.

This medieval tower is all that remains of the castle where Joan of Arc was held before being burned at the stake.

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

Despite centuries of architectural innovation, this French castle is being built like it's the 13th century.

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

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

An ancient ossuary in Rouen.

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

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

Figures of unknown origin carved into the surfaces of underground caves.

The sunken structure looks like the victim of a catastrophic flood.

This ancient theater was once the home of fierce gladiatorial battles.

A quaint medieval millhouse hovers surprisingly above the waters of the Seine.

A colossal sculptural lion built by the same man who designed the Statue of Liberty.

Ordinary art transforms to extraordinary thanks to locals' quirky sense of humor.