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

This cemetery features modernist statues as strange as its name.

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.

A beautiful but toxic fountain of mercury.

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.

This 19th-century island fort built to protect against an invasion by sea.

Eerily lifelike effigies stare at everyone who enters this sanctuary.

A spiral staircase connects the limestone cave with Pembroke Castle, birthplace of King Henry VII.

The Stonehenge of Wales is one of the grandest neolithic dolmens still in existence.

A medieval military street hidden beneath a charming French fishing village.

A perilous medieval road leading right into the sea.

A set of medieval stained-glass windows tell the gruesome story of a Viking siege.

This Norman castle has been the scene of assassination plots, religious persecution, and Nazi conspiracies throughout its long history.

These 1,500-year-old remains were found in a mysterious Anglo-Saxon mass grave.

A showcase of canine neckwear spanning five centuries of doggie fashion and function.

A Neolithic burial site built 1,000 years before Stonehenge.

The last resting place of the famous Native American princess is marked by a haunting monument.

A strange and ancient landscape hides the dark, cavernous remains of a 5,000-year-old flint mine.

This imprint of an old computer keyboard has a unique backstory.

A 4,500-year-old forest is slowly being revealed from beneath the sands of a Welsh beach.

Ruined 11th century abbey built by King David I in Edinburgh, Scotland.