This private skyway is filled with art and was expanded explicitly so that Hitler could appreciate a river.

The mummified relic of St. Antoninus.

The beautiful baroque library of the Hapsburg empire.

The largest Art Nouveau complex in the world is a Barcelona hospital with a 600-year-long history.

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

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

A former steam-powered mill from the 1800s is now an industrial park with a giant steel dragon slide.

A once-secret library of Freemason history in Catalonia.

Turntable anonymously received orphans, alms, and groceries.

A stately palace has been converted into the world's largest cannabis museum.

One of the most photographed buildings in a city known for its Modernista architecture is an abandoned squat.

Barça football fans might see the insignia hidden in a church window as a divine sign.

For one day every month Barcelona's Estació de França transforms into a market for model trains.

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.

The abandoned transportation hub hides within the Glasgow attraction it once served.

These rusting, skeletal structures are monolithic monuments to Glasgow's industrial past

Water-themed sculptures guard this unusually square structure.

The strikingly modern structure stands out among the brick buildings speckling the Harvard Business School's campus.

This parking garage was the site of what was - at the time - the largest cash robbery in history.

This beautiful architectural gem was the first waste treatment facility in Boston.

One of the indelible moments of the American Revolution took place at this location.

Sacks of potatoes, made of cast stone, mark where the iconic potato sheds once stood along the river.

The remnants of a Gilded Age elegance preserved in the middle of a stark modern tower.

The site where Charles Dickens gave his first public reading of "A Christmas Carol" in the US.