An Industrial Revolution-era public work that purified water using nothing but sand.

People escaping slavery may have hidden inside a corpse vault.

A nostalgic bibliographic gem.

The beloved statue at the National Academy of Sciences is oh so inviting to sit on.

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

This grand gothic brewery has been pumping out suds for over a hundred years.

Long-abandoned trolley tunnels just a mile away from the White House are turning into an art space.

The site of the climactic scene from the classic horror film is now a historic landmark.

One of Antoni Gaudí's least-visited works, an homage to Catalan patrimony that mixes neo-Gothic with Modernista forms.

A once grand hotel now sits eerily abandoned in the Catalan countryside outside Barcelona.

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

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

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

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

A beautiful but toxic fountain of mercury.

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

Amsterdam's Red Light District leaves little to the imagination.

Europe's official "zero-level," the equator of elevation.

Sail in for a ’dam good slice.

One of Amsterdam's narrowest houses is located right across the canal from its widest.

This bird-loving brewery is situated in an old municipal bathhouse, nestled beside a 300-year-old windmill.

A Portuguese Jewish cemetery filled with stunning grave monuments that date back to 1614.

Three connected pink triangles memorialize persecuted gays and lesbians throughout history.

This unattributed sculpture in Amsterdam is hiding in plain sight.