The narrowest street in the world is in danger of closing up entirely.

This old wooden funicular railway leads to a cemetery in the forest.

This outdoor library provides around-the-clock access to around 7,000 books.

This Nazi-built amphitheater was constructed on a sacred mountain site used by various German cults.

A kindergarten shaped like a cat with a tail that's a slide.

There has been an astronomical clock on this spot since the 14th century.

Today this former Nazi concentration camp stands as a poignant memorial to the victims of fascism, and a warning from history.

An ancient monastery, a locked room, and a mysterious book heist.

The birthplace of the builder of the Statue of Liberty is now a museum dedicated to his work.

The abandoned mine where American troops discovered a treasure trove of art and artifacts hidden by the Nazis.

Hiding the longest subterranean river known in the world.

This painted Lego bridge is the product of over a century of locomotive history.

The real life town from the Pied Piper of Hamelin has a mechanical puppet show that retells the famous story many times a day.

Striking occult iconography on a medieval German church.

The mass grave of the 27 victims of the "Vampire of Hanover."

Unchanged for centuries, this medieval library keeps its books in chains.

Constructed by forced laborers, this submarine pen is now a museum and war memorial.

The only microgravity tower in Europe can produce nine seconds of weightlessness.

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

The headless bodies of more than 1,300 guillotined victims of the Reign of Terror are buried here in mass graves.

What looks like a normal building is really a hollow facade hiding a subway vent.

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

Remains of Roman amphitheater hidden in a sleepy quarter of Paris.

Pitié-Salpêtrière was the dumping grounds for women who received the dreaded diagnosis of "hysterical."