The remains of the medieval wall that once surrounded Paris with ties to the construction of the Louvre.

The industrial architecture honors the laborers who brought the Exposition Universelle of 1900 to life.

Hidden away in a modern library are the remains of one of the 18th century's most celebrated wunderkammers.

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

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

A darkly erotic open-air statuary in Catalonia.

Medieval village and microminiature museum.

Wonderland replication in progress by one persistent man.

France's highest manned weather station is the only one still inhabited by meteorologists.

A once grand hotel now sits eerily abandoned in the Catalan countryside outside Barcelona.

Possibly the oldest church in Barcelona, this basilica has its pagan and Christian mythologies all mixed up.

18th century anatomical amphitheater of the Royal College of Surgery of Barcelona.

A once-secret library of Freemason history in Catalonia.

Barcelona's historic 19th-century sewers, whose foundations were laid in medieval times, are open to the public for exploring.

The studio of Catalonia's most famous theatrical designer remains exactly as he left it.

The former 1920s mansion of a prominent Catalan developer was used as the Soviet Embassy during the Spanish Civil War, and there is still a bunker below the garden.

Holy Grotto of Montserrat Monastery holds one of Europe's mysterious black Madonnas.