Two very green residential towers cleaning the air in one of Europe's most polluted cities.

An ossuary built in 1210 to collect bones from the Ospedale Maggiore of Milan.

The amazing wonder cabinets of a very rich Austrian archbishop.

19th century pendulum and a clock restored by a rogue group of guerilla artists.

An ornate sculpture of an ox in repose marks the former entrance to Nuremberg's medieval meat market, which was destroyed during World War II.

This historic covered bridge contains a series of macabre paintings that communicate one thing: everyone dies.

During World War II, the Nazis repurposed this beer cellar to store stolen artwork.

This strange sculpture depicts a giant nightmare hare crushing a man to death beneath its colossal bulk.

A castle from the Holy Roman Empire that hosts traveling youth... in its stables.

Around 100,000 bodies are buried below the medieval headstones, many of which mark graves with multiple bodies stacked up to 12 deep.

The hidden lair of an alchemist rediscovered in a 2002 flood.

Macabre astrological automaton clock dating to the late 1400s.

Two stunning 17th-century libraries and a cabinet of curiosities.

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

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

Medieval museum built over Roman baths that features famous tapestries of a lady and a unicorn.

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

This symbol of the French Revolution still stands where it was first planted during the 18th-century.

The castle was reconstructed to fit Kaiser Wilhelm II's romanticized vision of the Middle Ages.

This enormous "cloth of the conquest" depicts the Norman invasion of England in mind-blowing detail.

There has been an astronomical clock on this spot since the 14th century.

The tradition of open-air secondhand and antiquarian bookselling in Paris dates back to the Renaissance.

A surrealist timepiece by the artist of melting clocks.

A scandalous Art Nouveau collaboration that set Paris all atwitter at the turn of the century.