The 20-mile causeway that separates the Netherlands from the sea.

Live out your childhood construction fantasies in this one-of-a-kind gyrating hotel.

Take a stroll through the hellish grotesques of Hieronymus Bosch in this elaborate Netherlands church.

Birthplace of the "Cat in the Cradle" and a cluster of iconic, original windmills that let the Dutch tame the seas.

This centuries-old gothic monster has been converted into a sprawling, glossy bookstore.

A beautifully eclectic mix of architectural styles.

A gigantic statue of a Russian soldier watches over the ancient city of Plovdiv, a prominent reminder of the Soviet period in Bulgaria.

This ancient theater, restored in the 20th century, lives on as a venue for dramatic and musical performances.

UNESCO-protected ancient burial ground.

A mountain pass in Bulgaria's Balkan Mountains was the site of four historic battles.

The remarkably well-preserved final resting place of a powerful ancient Thracian king.

A magnificent, glittering building tucked into a tiny Bulgarian mountain town.

Ruins of an early Roman Christian church dating back to the 4th to 5th centuries.

An homage to the medieval rulers of Bulgaria.

Rock carving created 1,300 years ago 75 feet off the ground.

Mysterious standing stone formations looks like a petrified forest.

This massive stone and steel monument remembers a century-old battle that cost thousands of lives and nearly destroyed the city.

This once-grandiose behemoth of a monument from the socialist era now sits quiet and closed up.

In a long-dry lake bed, these groups of massive stone pillars bear a striking resemblance to clumps of everyone's favorite fungi.

An abandoned saucer-shaped monument in the Bulgarian mountains.

An ancient Thracian city where fire rituals were practiced on the largest cult altar in Southeast Europe.

This massive concrete sculpture is said to be the heaviest communist monument on Earth.

This snug medieval lane is one of the narrowest streets in the world.

Once an opulent Art Nouveau hotspot, the Constanța Casino is one of Romania's most beautiful abandoned buildings.