Twelve grave-faced marble angels guard the emperor's tomb at Les Invalides.

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.

A Parisian exterminator's shop whose storefront displays stuffed rats hanging from traps.

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

An overlooked marble is the only surviving in-place 18th century example of the measurement that changed how we define distance.

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

Paris' fabled purveyor of exotic taxidermy and natural history curios.

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

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

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

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

Stroll through retro Paris while shopping for old postcards and discarded doll heads.

A haunting collection of the vanished and disappearing natural world.

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

Paris' linguistic guide to the international language of, yes, love.

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

Abandoned railway line circling the city of Paris.

An early work of the architect Álvaro Siza Vieira, these swimming pools blend into the beach.

The abandoned remains of this Victorian brewery hide crumbling bowling alleys and rusting industrial gear.

A tour of underground Berlin takes you through WWII air raid shelters and tunnels used to smuggle people out of East Germany.

This futuristic German spa allows visitors to float in a pool of salt water while zoning out to underwater techno.

A slice of central Asia in the middle of Berlin.

This classic restaurant features a gorgeous mirror-clad room.

The largest free-standing aquarium in the world resides in a hotel lobby in Berlin.