While not Turkey's deepest underground city, this subterranean redoubt is the largest.

In this historic region, homes and churches are carved into ancient volcanic rock formations.

A surprising and unconventional mosque hidden in the subsoil of Istanbul.

This historic hotel preserves many treasures from its heyday hosting passengers on the Orient Express.

This former Imperial summer home was the venue for a royal slap that once echoed 'round the world.

This half-forgotten fortress was constructed by the Ottoman conqueror of Constantinople.

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

Ancient serpentine sacrifice holder that has stood the test of time.

A fountain standing as a testament to a doomed alliance.

Climb a steep hill in 90 seconds on the second-oldest urban subway in the world.

A gorgeous, Gaudí-esque stairway constructed in the city of Istanbul by a prominent Jewish family.

An Art Nouveau gem hides beneath the layers of city grime obscuring this once-magnificent building.

The most metal church of all time was built in record time because of an international, pious pissing contest.

Step back in time and witness the fall of Constantinople.

This remarkably well-preserved Egyptian plinth is pretty well traveled for a giant piece of stone.

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

The ornate miniature mansions were built for Istanbul's feathered residents.

A marvel of Byzantine engineering under the streets of Istanbul.

"Holy Wisdom" overlooks the city that became its namesake.

A vast Communist-era exhibition space in the Bulgarian capital.

A striking symbol of the Bulgarian capital.

A collection of art from Bulgaria's time as a communist state.

A Bulgarian monument to Russia's role in World War II is constantly being painted in pop culture protest.

Dedicated to the European Sinti and Roma peoples murdered during the Holocaust.