A Turkish monastery tucked into a cliff wall.

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

Ottoman-era houses lovingly preserved.

Hidden just under the waters around this Turkish island are the ruins of a once great city.

A Turkish ghost town abandoned in a population exchange with Greece.

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.

A beautiful mountaintop mausoleum devoted to a Montenegrin icon can't seem to make everyone happy.

A curious cave church carved into the face of a cliff.

These historic defenses ensured the city within would survive hundreds of years of occupations, sieges, and invasions.

Hundreds of thousands of concrete bunkers crowd Albania long after the fall of Communism.

An Albanian monument to the Communism that nearly crushed the country is now a crumbling wreck.

A naturally beautiful, seemingly bottomless deep blue spring.

The hallucinogenic center of the ancient world.

Monasteries on rock pillars, once accessible only by frayed ropes.

The bizarre rocks of Đavolja Varoš are the result of natural erosion.

Operating continuously since 1903, this "milestone" facility transformed the regional economy.

A towering spire in the fortress of Kalemegdan honors the victorious armies of Serbia during their struggles against the Ottoman Empire.

Sealing couples' love since just after World War I.

In the hinterlands of Serbia's capital, a giant reflective mushroom houses an aeronautical history of the former Yugoslav republic.

Curious carved stones and bird-like wings are a symbol of Serbian resilience and freedom on the site of a WWII massacre.

Serbia's master of striking memorials erected a forgotten, hidden gem in the hills outside a tiny village.