Ancient barriers wrap around what was once an important stop along the Silk Road.

Light still pours into the ruins of this ancient Islamic observatory.

This basilica is dedicated to two saints, one of which is known more for his famous quote.

This one-of-a-kind Roman arch is not actually dedicated to the god Janus.

A fortified medieval basilica on the Caelian Hill preserves extraordinary frescoes and one of Rome's most beautiful hidden cloisters.

This small, ancient Roman temple was once a church dedicated to the patron saint of penitents.

The ruins of the largest gladiatorial school in Rome are largely hidden under modern streets.

A detailed pachyderm supports Rome's smallest Egyptian obelisk.

This ancient market is often referred to as the oldest shopping mall in the world.

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

A forgotten "city of water" underneath the famous Trevi district.

This hidden Art Nouveau courtyard is an opulent feast for the eyes.

Once designed as the entrance to Buckingham Palace, this structure was later relocated and stands as a victory arch.

A clock designed to run with perfect accuracy for 10,000 years.

Kings Cross Station pays tribute to its role in Britain's best-selling book series.

Thanks to an acoustic anomaly, this unassuming stone bench has heard one hundred years of secrets.

Spain's largest bullring offers a unique glimpse at architecture and history.

This former charity hospital for elderly clergy houses some of Seville’s most treasured paintings.

The world's largest wooden structure.

A curious carving hides on a marble column in the Mosque-Cathedral of Córdoba.

Spain's unique Mosque-Cathedral has been a place of worship for both Muslims and Christians throughout its history.

Remarkably intact Roman stone fortifications have encircled this Spanish city for centuries.

Shelves covered in dozens of coffee bean varieties fill this North End institution.

The tomb of Increase Mather and his son Cotton, influential colonial ministers obsessed with the occult.