France's national museum of scientific and industrial instruments.

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

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

A museum of carnival attractions where you are invited to ride the rides.

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

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

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.

The vast, legendary catacombs hold secrets much stranger than stacked bones.

This ex-prison dates back to the Middle Ages.

Where visitors can play more than 150 retro arcade games.

One of the world's largest amber altarpieces is housed within this reconstructed medieval church.

An extensive network of secret tunnels was created for the Nazi project "Riese."

Old cabin-like homes in a rural community untouched by time are known for their vibrant and colorful window shutters.

A collection of modern and vintage pinball machines housed in the cellar of a 15th-century building.

This picturesque castle in Poland has a rich history, including a legend that ties it to lost Incan treasures.

In northeastern Poland, the ruins of a massive granary overlook the Narew River.

One of the world's largest collections of medieval religious art.

The Polish capital has had a deep connection to its guardian mermaid for almost a millennium now.

St. Mary's features one of the most spectacularly decorated cathedral interiors in all of Europe.

The former Nazi concentration camp stands as a museum to remember this dark chapter in European history.

A fairytale Gothic castle steeped in Nazi treasure conspiracy theories.

A program of Communist glamour in post-Stalin Poland is preserved in this colorful museum.

This Polish cave has housed a dragon, a brothel, and an enduring legend.