An Orthodox church built by the Royal Family for the Russian community exiled to the French Riviera.

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

A mud statue stands in the dark tunnels, seeming to hold up the weight of the city above.

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

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

This off-limits room in the National Library of France is home to a system of pneumatic tubes, once used to bring books to readers.

This massive blockhead sculpture hides an entire library.

Hundreds of hidden passageways weave through the old quarters of Lyon.

The curious underground history of keeping Paris clean.

Sacked by Vikings, blessed by William the Conqueror, dealt a fatal blow during the French Revolution—these ruins have seen it all.

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

This French ghost town from World War II has been left as a memorial to German atrocities during World War II.

A subterranean shelter remains intact under a busy Paris railway station.

"In the Dark" Restaurant in Paris offers dinner in complete darkness.

This museum houses an imposing skeleton army comprised of creatures great and small.

France's national museum of scientific and industrial instruments.

This Parisian café sticks almost exclusively to an ethos of reuse.

A favorite Paris bar for U.S. expats, including Gershwin, Hemingway, and James Bond.

A museum of magic with a Sadist past.

A counter-culture bookstore like no other hides in plain sight in the streets of Paris.

A haunting collection of the vanished and disappearing natural world.

A horde of stray cats rule in this secluded Parisian cemetery under a bridge.

This charming cobbled backstreet is the most Instagramable street in Paris, much to the residents' annoyance.

This massive library holds what was once the largest book collection in the world.