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.

This famous city eluded concrete detection until the early 2000's.

These tombs were built into the side of a mountain so that ancient angels could reach the dead.

A marvel of Byzantine engineering under the streets of Istanbul.

Roman ruins top white travertine terraces formed by ancient hot springs.

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

Only a handful of these gorgeous historic waterwheels remain but even those are in danger from local unrest.

Ruins of the Crusaders' headquarters.

Mysterious lost empire of the Silk Road.

A bathhouse from the second Islamic Caliphate decorated with frescoes of constellations, pleasure gardens, and naked ladies.

The world's oldest map of the holy land gives us a fragmented glimpse of the layout of biblical nations.

Gladiators battle and chariots race daily in the ruins of a once-great Roman city.

A more chill but no less fascinating version of the famous ancient archeological site.

An ancient capital city that dates back 3,000 years old and harbors many surprises.

Bombed and burned, this monument represents the often controversial ties between Vietnam and Cambodia.

Khmer Rouge's high security prison.

A quick and simple solution to Cambodia's train woes.

This impressive clifftop temple was once at the center of an international custody dispute.

This Cambodian riverbed is covered in ancient Hindu carvings both above and below the waters.

5,000 skulls in memorial to those killed by the Khmer Rouge.

A battle between nature and architecture in the Cambodian jungle.

Thousands of narrow alleyways in an ancient medieval city make up what's believed to be the world's largest car-free urban zone.

The other side is actually green.