Possibly the oldest church in Barcelona, this basilica has its pagan and Christian mythologies all mixed up.

A once-secret library of Freemason history in Catalonia.

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.

Turntable anonymously received orphans, alms, and groceries.

The 2,000-year-old Roman columns are hidden within Barcelona's Gothic Quarter.

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

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

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

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.

The crypts of Capuchin friars decorated with the bones of over 4,000 friars, including an entire "crypt of pelvises."

The underground sanctuary of a mysterious ancient cult.

This curious, round temple is the oldest still-standing marble building in Rome.

The yawning maw of this pagan visage is said to bite off the hands of liars.

This Roman arch marks the location of an ancient gate at one of the Seven Hills of Rome.

An alchemist's "magic door" stands in the middle of a Roman park.

The oldest stone house in Paris was built by its most famous alchemist.

An underground paleo-Christian burial site composed of the remains of several cemeteries and Basilicas.

Skulls from the plague of 1656 and World War II bombings are united by a cult dedicated to caring for the ancient dead.

The vast, legendary catacombs hold secrets much stranger than stacked bones.