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

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

The ruins of a Duke's bucolic dream, where camels once roamed alongside "Dutch" windmills and faux-Italian vineyards.

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

A mud statue stands in the dark tunnels, seeming to hold up the weight of the city above.

A Dutch museum preserves the world's most diverse collection of vertical slices of soil.

The European equivalent of the Hollywood Walk of Fame is slowly spreading over the entire city.

A curious collection dedicated to human and animal anatomy and pathology.

The first crematorium in the Netherlands.

The ruins of a medieval castle that once belonged to one of the most powerful families in the Netherlands.

This former hydraulic laboratory is now a woodland dotted with old engineering models.

A maze of crosses marks the jumbled divide between the Netherlands and Belgium.

The largest free-standing aquarium in the world resides in a hotel lobby in Berlin.