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

Construction of Barcelona's iconic (but controversial) church is expected to be completed in 2026—a century after the death of its architect.

The "Prussian Versailles" that is as glorious today as when it was built.

Giant marble phalluses on the sacred island of antiquity.

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

Macabre astrological automaton clock dating to the late 1400s.

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

This abandoned coliseum was intended to be a grand Nazi rallying ground before it was halted at the end of the war.

This eccentrically decorated palace is replete with grottoes, fountains, tunnels, and caves.

An underground city of salt.

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

Castle and secret tunnel withstood a siege until its owner was killed by the Holy Roman Empire, while sitting on the toilet.

Stunning Roman aqueduct used for 1900 years and still standing.

Paranormal experiences in this hotel's room 217 led Stephen King to write "The Shining."

Historic medieval fortified city in southwest France.

The prototype for all Nazi concentration camps.

This one-time health resort was used by aristocrats around the world.

Old Danish castle immortalized in Shakespeare's Hamlet.

A king's pet, a pope's gift, an artist's inspiration, and a tragic shipwreck.

A tiny church on a man-made island in the Adriatic.

Bizarrely shaped rock formations in northwest Bulgaria come with equally bizarre legends.

A chain of 16 wondrous lakes formed as a result of the gradual dissolution of soluble layers of bedrock.

Daily jousts and dancing barrel makers in Munich's mechanical clock.

Giant topiary dog once played a role in a terrorist bomb plot.