A 1940s nuclear reactor built to produce plutonium for the Manhattan Project.

An obscure waterfall that looks like something out of a fantasy novel.

These magnificently sculpted tombs represent Spain's own legendary version of "Romeo and Juliet."

This multi-level, aqueduct-footbridge hybrid is one of the greatest works of the Spanish Renaissance.

The oldest operating roller coaster of its kind, the Montaña Suiza has been thrilling riders since 1928.

Hundreds of stone stairs and a winding medieval bridge connect this haunting Spanish island to the mainland.

A historic Christian shrine nestled within a spectacular natural cave.

A micro-communist utopia in Spain with no police, no mortgages and full employment.

This historic black and white monastery is one of Bulgaria's architectural jewels.

An abandoned saucer-shaped monument in the Bulgarian mountains.

A Lycian city of the dead carved into the hillside of Southern Turkey, complete with startlingly human touches.

The best-preserved ancient theater in the world.

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

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

A deadly ancient portal to the underworld.

This hunter-gatherer architecture is believed to be the oldest religious complex known.

Mausoleum and Holy Seat of Antiochus I of Commagene.

A Turkish monastery tucked into a cliff wall.

Unfinished monument to peace between Turkey and Armenia, long divided by a dispute over genocide.

The observation point offers spectacular views of the ruins of an ancient Armenian capital.

An abused and forgotten metropolis, abandoned for centuries.

An ancient church on a tiny island covered in stone bible stories has controversially been turned into a museum.

A focal point of the Cuban government, this city square features one of the most iconic images in the country.

Once a favorite playground for the world's most illustrious guests, Hotel Nacional's secret weapons and bunkers reveal a more menacing side to its golden history.