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

Even France's favorite aquatic son Jacques Cousteau, couldn't get to the bottom of it.

The oldest and smallest cemetery in Paris is only open to the public twice a year.

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

A unique underground structure dating back to the 1st century BC.

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

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

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

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

An unusual number plate signifies the building's saucy past.

The remains of 40,000 humans, meticulously stacked in a tiny churchyard shed.

Since ancient times these picturesque springs have been providing bathers with healing waters.

Lyon, France is home to two remarkably preserved Roman theaters, one dating back to 15 BCE.

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

A medieval military street hidden beneath a charming French fishing village.

This crumbling, graffiti-covered mansion sits tantalizingly within reach of the proletariat who frequent a public park.

This ancient Roman bridge and aqueduct still stands as a wonder of classical construction.

BDSM, 1930s Style.

This innocuous-seeming building is actually the remnant of a major wartime brothel.

A mosaic-tiled house built by graveyard sweeper Raymond Isidore.

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.

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

A self-built mosaic house and sculpture park.