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.

Step inside the first house designed by Catalan architect Antoni Gaudí, considered to be one of the first Art Nouveau buildings.

This Funeral Hearse Museum represents the finest in cadaver transportation.

One of Europe's largest supercomputers is housed inside of a disused chapel.

This former Baroque palace on the historic Calle Montcada is a rare architectural gem and was once the meeting place of an aristocratic scholarly society.

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

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

Mountain where Jesus was first tempted by the Devil.

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

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

This cemetery features modernist statues as strange as its name.

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

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

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

A beautiful but toxic fountain of mercury.

This once-grand marvel of engineering is now 41 miles of scenic hiking trail.

An unusual round church in London with a Templar past.

The perfectly preserved underground rooms where Churchill plotted the war against Germany.

Winding paths and ramshackle mausoleums in the oldest "park cemetery" in London.

This churchyard arbor is surrounded by hundreds of gravestones placed there by author Thomas Hardy.

A 300-year-old tea shop that brought tea to the English people, not to mention the Queen herself.

One of the few remaining casualties of the London Blitz, this destroyed church has become an enchanting public garden.

This ornate Victorian marketplace was the setting for Diagon Alley and the Leaky Cauldron in the Harry Potter films.

London's creepiest cemetery was once the site of dueling magicians and mobs of stake-carrying vampire hunters.