This sprawling Belgian fort was once thought of as impenetrable but now it is simply an abandoned relic.

Hiding the longest subterranean river known in the world.

A Belgian self-built tower inspired by the Bible and ancient civilizations.

Sculpture designed by Turkish artist and professor Mehmet Ali Uysal.

This abandoned Belgian fortress is purposefully being left to nature.

A once-magnificent collegiate church is falling into ruin.

A marble Lucifer hides behind the pulpit of this Gothic cathedral.

This spire on an Austrian mountain is wrapped around a long spiral slide and has the world's tallest wooden observation deck.

The world's oldest known salt mine.

An Austrian house of bones filled with hundreds of intricately painted skulls.

Swarovski's museum/theme park is a fake diamond fantasia overlooked by a giant.

This splendid tomb may look fantastic, but there's nothing in it...

Bizarrely-painted church designed by a controversial artist.

The shell of a medieval fortress offers commanding views of the area it once protected.

Resting in the middle of an Austrian river, this sci-fi "island" lets people cross in neon style.

Austria's ultra-modern art museum looks like Cthulhu's giant alien heart.

Double spiral staircases are not unheard of, but they are very rare.

Each year melting snow turns these Austrian hiking trails into scuba zones.

This series of underground chambers contains Europe's largest subterranean lake that is slowly drained each day.

Giant WWII fortresses that fired 8,000 rounds a minute, now home to thousands of pigeons.

This cubist Vienna church building looks like some sort of alien stonehenge.

The topiary maze at the Habsburg hunting lodge.

Apartment complexes built into huge repurposed natural gas tanks.

Op-art architecture designed for happiness.