The intricate geometry inside this Baroque-style dome was designed to create "terror of the human soul."

Two very green residential towers cleaning the air in one of Europe's most polluted cities.

A striking 12th-century-style castle built on the edge of a cliff.

This museum tells the story of lake dwellers from 4000 BC to 850 BC, replete with stilt houses and goats.

In this brilliant blue spring in Germany, legend has it that a sad mermaid learned to finally laugh.

This strange statue features a bust of the famous composer plopped atop a woman's body.

The medieval network of canals hidden underneath the city is opened to explorers each year.

An opulent monument built for a duke who donated his entire fortune to the city of Geneva.

The world's largest second-hand graces a clock surrounded by constantly changing flower beds. 

An 11th-century fortress home to one of the most influential royal houses in medieval Europe.

The water-powered carriages are long gone, but for over a century this funicular railway has been the fastest route to the highest point in Paris.

The oldest and smallest cemetery in Paris is only open to the public twice a year.

With a history of slaughter, feast, and famine, remnants of the pork industry decorate this area--even its church.

A 16th-century fountain marks the site of a lost medieval cemetery once filled with mass graves.

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

This 18th-century sundial was designed to calculate the date of Easter each year.

The video game-themed street art first popped up in the French city.

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

Natural history museum celebrating biodiversity with a parade of taxidermy animals.

The tradition of open-air secondhand and antiquarian bookselling in Paris dates back to the Renaissance.

The very center of Paris is surrounded by a bevy of strange good luck rituals.

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

A beautiful Kentish country intersection hosts a tasty little photo spot.

This curious Tudor-era castle was once home to Napoleon's greatest adversary, the Duke of Wellington.