A marvel of Byzantine engineering under the streets of Istanbul.

Five sultans and their families rest in eternal splendor, teeming with overlooked beauty and drama.

No one is sure if the world's fourth-largest diamond had been found in a rubbish heap or was pawned by Napoleon's mom.

1513 Turkish world map, full of European state secrets and snarky commentary.

This column celebrates Rome's victory over the Goths, and may be the oldest Roman monument in Istanbul.

An ancient Roman temple first dedicated to an emperor's wife is now a church.

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

Ancient ruins hidden underneath a modern luxury hotel.

The remains of this 2nd-century Roman temple are incorporated into a modern building.

This small church was constructed over the ruins of three temples dating to the Roman Republic.

This is the oldest Roman-made bridge in the city still existing in its original state.

The "Greatest Sewer" of ancient Rome is one of the oldest sewer systems in the world, and is still in use.

The only remaining arch of the oldest stone bridge in Rome stands in the middle of the Tiber.

Remnants of a 4th-century church with vivid medieval frescoes and ancient sarcophagi hide beneath this old basilica.

A small cell adjacent to this church was once the bedroom for a renowned saint.

Brass cobblestones mark the places where individual Holocaust victims were taken from their homes.

Hidden beneath the famous St. Peter's Basilica is a veritable city of the dead that may hold the remains of St. Peter himself.

Drink wine handed out through a window as part of an age-old tradition.

A large grotto built in the 16th century amidst Florence's famous Boboli gardens.

The gondolas let you glimpse a bird's-eye view of one of the world's highest capital cities.

Since the 1700s, this old barrel has acted as a post box that relies on the kindness of travelers.

An idyllic swimming pool tells the tale of Hemingway's trouble with women.

A monument celebrating the idea of reaching inaccessible places.

The former home of the Nobel Prize-winning American writer is now home to dozens of six-toed cats.