Stay overnight in the world's most sleekly designed rabbit hole, free of charge.

This massive concrete sculpture is said to be the heaviest communist monument on Earth.

This once-grandiose behemoth of a monument from the socialist era now sits quiet and closed up.

One of the largest wooden constructions in Europe was meant to be a casino, but soon became an orphanage.

A sixth-century Byzantine cistern hidden below a modern carpet store.

Turkish adventurer antiques in the heart of the Grand Bazaar.

A marvel of Byzantine engineering under the streets of Istanbul.

This eatery re-creates dishes from the Ottoman Empire so you can dine like a sultan.

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

Naturally protected city carved from a rock face.

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

The deepest underground city in Turkey was designed to protect 20,000 people behind massive stone doors.

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

This ancient subterranean megalopolis is known for the hot oil traps that were installed above each of its entrances.

Roman ruins, natural methane fires and a lovely beach.

Flames have been burning within this mountain for more than 2,500 years.

Once described as the "Eagle's Nest," this ruined ancient city rests on a remote mountaintop.

The best-preserved ancient theater in the world.