A derelict bit of transportation infrastructure hidden in the woods.

A hidden gem on the lawn of the U.S. Capitol Building designed by landscape architect Frederick Law Olmsted.

Enormous piles of historically significant stones, dumped by Congress in a forest, and abandoned for 60 years.

A tiny piece of the Moon is embedded in this stained glass masterpiece.

With over 100 jam-packed rooms to explore plus elaborate tea services and events, the Mansion on O Street is a hidden treasure.

One of the few remaining ancient aqueducts described by Frontinus as 'the most solemn testimony of the Empire.'

Hundreds of stone stairs and a winding medieval bridge connect this haunting Spanish island to the mainland.

Two towns are linked by an engineering marvel masquerading as a boring suspension bridge.

This 19th-century fountain's namesake animal isn't what you think it is.

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

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

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

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

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

This cemetery features modernist statues as strange as its name.

A beautiful but toxic fountain of mercury.

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.