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

Walk in the footsteps of the gods and straddle two tectonic plates.

Every equinox this Maya pyramid puts on a spooky ancient light show.

Hear the quietest sound from across the dome.

An Irish church where an 800-year-old mummy is reaching out of his coffin as if to shake hands.

The 953 windows undoubtedly make this the world's most beautiful screened porch.

This massive subterranean fissure is known as the "underground Grand Canyon."

The longest hedge maze in the world.

Iceland's largest lagoon is home to stunning multi-colored icebergs.

A marvel of Byzantine engineering under the streets of Istanbul.

A fantastic castle rumored to be the birthplace of King Arthur.

The gorgeous library at Trinity College is home to remarkable treasures of Irish identity.

This 14th century castle withstood years of battle but is now sliding off the hills into the Danube.

Destroyed Serbian army headquarters sits half crumbled in a largely revitalized Belgrade.

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

A small memorial to a deadly night in Serbia that shook the world before it was swept under the rug.

Religious structure known for its sculpted details.

Romania's triumphal arch looks out over Bucharest.

A giant portrait of an ancient king looms over the river.

The growing stones of Romania.

Thousands of coffin-like pillars make up this controversial Holocaust memorial.

Cold war icon and onetime "hole" in the iron curtain.

The bunker where Hitler took his own life is hidden under this deliberately ordinary parking lot.

Berlin's old airport.