Underneath a chalk hill in France lies what Hitler hoped would be the underground launching base for a devastating new weapon.

The former home of a trailblazing surrealist is still filled with haunting dolls and grim paintings.

A Cuban artist reclaimed his impoverished neighborhood as a dreamy folk art kingdom.

Open since 1938, this diner has kept its fountain, malt machine, and all the gum that greasers left beneath its counter.

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

Op-art architecture designed for happiness.

A neo-Gothic mansion now houses a wunderkammer of analog tech that hits on all the senses.

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

An exquisite Baroque church in Vienna's old town.

Built to give thanks for the passing of a plague epidemic, this cathedral is a one-of-a-kind architectural hybrid.

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

The beautiful baroque library of the Hapsburg empire.

Gaudí's famously flamboyant architectural style is on full display in this sprawling park and garden.

One of Antoni Gaudí's most classic buildings is well-known for its "dragon-back" design.

Week-long block party becomes a competition for the best decorated street.

Construction of Barcelona's iconic (but controversial) church is expected to be completed in 2026—a century after the death of its architect.

These secluded and abandoned anti-aircraft defenses have become one the most scenic viewpoints in Barcelona.

Bask in the 19th century grandeur of an eccentric Freemason millionaire's private gardens.

This semi-secret storybook hedge maze is also the oldest garden in the city.

This cemetery features modernist statues as strange as its name.

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

Two small chapels housed inside an ancient tree.

This museum houses an imposing skeleton army comprised of creatures great and small.

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