Catacombs turned marketplace.

This ornate Victorian marketplace was the setting for Diagon Alley and the Leaky Cauldron in the Harry Potter films.

Spitalfields' regal market goat is said to be a monument to the area's migrant population but most are pretty sure it's just a goat.

Preserved behind glass, the ruins of a medieval building used to store human remains.

Once designed as the entrance to Buckingham Palace, this structure was later relocated and stands as a victory arch.

Kings Cross Station pays tribute to its role in Britain's best-selling book series.

Two awe-inspiring globes from the reign of the Sun King hang from the ceiling of France's national library.

Paris' linguistic guide to the international language of, yes, love.

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 narrowest street in Paris.

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

This life-size replica of the Statue of Liberty's flame is also a tribute to Princess Diana by default.

In this Parisian garden's grotesque former life as a zoo it fed exotic animals to the bourgeois and put humans on exhibit.

A French artist's vague warning hangs over Belleville.

This off-limits room in the National Library of France is home to a system of pneumatic tubes, once used to bring books to readers.

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

Once the longest swing bridge in Europe.

Thousands of tourists and residents in Cologne affix "love padlocks" on the Hohenzollernbrücke Bridge to symbolize their affections.

The largest bell in the world has been swinging high above a Cologne cathedral for almost a century.

This small condiment museum makes all manner of mustards with a 200-year-old mill.

This structure atop a craggy cliff in one of the most beautiful parks in Paris boasts an unbeatable view.

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

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

One of Jeff Koons’s largest sculptures and his first commemorative work.