A hidden bathhouse in one of Stockholm's oldest and busiest neighborhoods is open to anyone who can find it.

Dine on food tied to Viking history and legend at this subterranean Stockholm spot.

Sweden's first nuclear reactor, R1, can be visited 25 meters underground.

Lyon's "Wall of the Silk Weavers" is one of the largest murals in Europe.

Monsters and miniatures are held in equal esteem at this wondrous museum.

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

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

If you look closely at the oldest bridge in Paris, you might notice hundreds of ugly faces leering from its sides.

A modern-day Lascaux cave underneath Paris.

Skeletons, medical artifacts, and other obscure and precious treasures all displayed in a wooden pharmacist's cabinet.

Original cabaret handbills, posters, and feline artwork are a purrfect highlight of the Museum of Montmartre.

The history of radiation is on safe display at the foot of a radiation science laboratory.

An "inside out" building in the historical center of Paris, featuring Europe’s largest modern art museum.

High atop the Eiffel Tower is a small apartment built exclusively to entertain the science elite and make the rest of Paris jealous.

A scandalous Art Nouveau collaboration that set Paris all atwitter at the turn of the century.

A haunting collection of the vanished and disappearing natural world.

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

Notorious artist squat renovated into legal studios.

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

France's most famous cemetery, with some of its most curious tombs.

This modern, multimedia museum takes visitors on an evolutionary journey back in time.

This only museum in the world dedicated solely to mushrooms.

A tribute to an iconic act of defiance from the Croatian War of Independence involving a small car.

This military tunnel dates back to World War II and provides visitors with a little wartime adrenaline.