A 19th century artistocrat's mansion, turned into a library, hidden in a modern library.

The subterranean "lake" below the Paris opera house inspired the Phantom of the Opera's lair.

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

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

Noirish green neon marks the location of one of the oldest private detective agencies in France.

The oldest stone house in Paris was built by its most famous alchemist.

One of the last umbrella repair shops in Paris upholds the nostalgic art of umbrella making.

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

Abandoned railway line circling the city of Paris.

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

The narrowest street in Paris.

Statue of death tucked in the University of Medicine courtyard.

This iconic Parisian bookstore doubles as the "Tumbleweed Hotel" for traveling writers.

The recovered bones of martyrs slaughtered during The Terror are on display in this church crypt.

The Sun King's military models, saved from the Louvre.

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

Walk upon the mementos of the dead in this Parisian courtyard paved with medieval tombstones.

This demon atop the Notre-Dame de Paris looks utterly bored.

This former “ante-chamber to the guillotine” has been refurbished to illustrate the brutal history of French justice.

The approximate spot where the last Grand Master of the Knights Templar was put to death.

The celebrated eyesore created by starry-eyed sweethearts.

A massive sculpture of an obscure Celtic coin that has become a symbol in Slovakia.

The inverted pyramid design of this station stands out amid Bratislava's urban landscape.

Poland's oldest surviving university building holds a wealth of historic science instruments including some used by Copernicus.