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

Ancient serpentine sacrifice holder that has stood the test of time.

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

The world's most elegant power lines.

Wonderland replication in progress by one persistent man.

Houses are tucked between, on, and underneath giant boulders.

An enchanting monastery in the Spanish countryside appears lifted from a fantasy tale.

These striking ruins of a castle resemble a ship's stern.

One of Antoni Gaudí's most classic buildings is well-known for its "dragon-back" design.

This ancient Spanish castle sits high upon a hilltop.

This medieval fortification has weathered the centuries better than any other structure from its era.

Archaeological site of Iberian people ruins.

Now a pile of ruins and rubble, this was a lovely mountain town as recently as 1960.

A museum dedicated to the Spanish occult in a town that was terrorized by the Inquisition.

Geological storytelling.

"Alleys of Stone" form natural mazes in Spanish natural park.

Cliffside houses provide a spectacular view from their perch above the valley.

An ancient Egyptian temple in the middle of Madrid, Spain.

One of the few remaining ancient aqueducts described by Frontinus as 'the most solemn testimony of the Empire.'

Notorious artist squat renovated into legal studios.

France's national museum of scientific and industrial instruments.

This former “ante-chamber to the guillotine” has been refurbished to illustrate the brutal history of French justice.

An abandoned relic of the city's failed experiment with below ground transit.

The longest navigable underground river in the United States is home to countless blind and albino animals.