The enormous 250-room Vanderbilt mansion conceals hidden doors and secret passageways.

This city square is home to Amsterdam's oldest church and a statue honoring Dutch sex workers.

A little sculpture with a large wardrobe.

This collection of over 50 murals celebrates Brussels' rich history as the self-proclaimed home of the "comic strip."

Three floors of musical instruments coming from all corners of the world and hundreds of years of musical history.

Climb inside an iron crystal magnified 165 billion times its normal size.

Lyon's miniature version of the Eiffel Tower was built as a statement in support of French secularism.

An icy lake tucked among jaw-dropping alpine panoramas.

This ancient theater was once the home of fierce gladiatorial battles.

Even France's favorite aquatic son Jacques Cousteau, couldn't get to the bottom of it.

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

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

This ancient Roman bridge and aqueduct still stands as a wonder of classical construction.

Historic medieval fortified city in southwest France.

An "inside out" building in the historical center of Paris, featuring Europe’s largest modern art museum.

A Parisian cemetery where the elite and artistic are interred beneath an eccentric collection of tombs.

A museum on the history of Paris, including a set of Napoleon's toiletries, Proust's room, and relics of the Revolution.

A cablecar-station-turned-revolving restaurant with 360-degree views of the Swiss skyline.

An ancient knight and a symbol of the city's freedom.

Longest fortress system anywhere in Europe.

When Croatia became independent of Yugoslavia, the splintering governments left behind these concrete tunnels.

A chain of 16 wondrous lakes formed as a result of the gradual dissolution of soluble layers of bedrock.

Dr Barnes' $25 billion private collection of art and arboretum.

America's most famous museum of medical oddities is home to the remains of Albert Einstein's brain.