Stockholm's decidedly unique public transit system has doubled as the world's longest art gallery for decades.

César Baldaccini's iconic sculpture of a 40-foot thumb stands tall in La Défense.

Abandoned castle was once a grand example of Russian privilege, built exclusively to win a frivolous bet between noblemen.

This ancient Spanish castle sits high upon a hilltop.

A South African bar inside a hollowed out 6,000-year-old Baobab.

When the rain falls, this drain system turns into a musical instrument.

Silent memorial, using bone conduction to take people back to the Dresden Bombing of 1945.

Stunning wetlands ablaze with pink lotus flowers.

A corkscrew-shaped barn in Ireland.

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

Europe's second largest ossuary wasn't discovered until 2001.

Located right outside of Vienna, visitors can take in expansive views of an ethereal forest.

A spherical "micro-nation" in the heart of Vienna.

Op-art architecture designed for happiness.

Giant WWII fortresses that fired 8,000 rounds a minute, now home to thousands of pigeons.

The crypt of the imposing Stephensdom holds royal intestines and thousands of skeletons.

This former insane asylum now holds a pathological anatomy museum.

Baroque celebration of the end of the Great Plague of Vienna.

The house that took a plunge into the facade of a museum.

The Habsburgs' royal collections of art and wonders.

Synagogue built from the stones of Solomon's temple contains the golem of Prague.

Macabre astrological automaton clock dating to the late 1400s.