These 12th-century catacombs carved into a mountainside were a place of mysticism and Christian hermitages.

Invisible bridge parts the waters, invoking biblical images and preserving the ominous feel of an ancient moat.

This "floating railway" is one of the world's oldest monorail systems.

This jaw-dropping 19th-century bridge uses its reflection to form what appears to be a perfect circle.

Martin Bricelj Baraga's monolithic sculpture measures the blueness of the sky, and changes color to match it.

A miniature train-ride through Slovenia's largest cave system.

A measuring mishap led to these artificial stone ladies losing their stomachs.

An industrial relic is now one of Europe's most staggering art spaces.

A relic from the Industrial Revolution got a second life as a contemporary performing arts space.

A cemetery for Slovenian notables also serves as a popular recreation space.

This abandoned Slovenian military barracks has become a punky-political art squat.

The walls of this medieval church are covered with perfectly preserved frescos, including a pristine example of the Dance of Death.

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

This city square is home to Amsterdam's oldest church and a statue honoring Dutch sex workers.

Incredible labyrinth of man made caves.

The ruins of this Dutch castle sit atop a labyrinth of once-secret tunnels and passageways, dating back over 900 years.

This historical Netherlands fortress still bears the marks of its single siege.

Go inside a revolutionary, cage-less zoo where 70 species of monkeys roam free alongside their human visitors.

This collection of over 50 murals celebrates Brussels' rich history as the self-proclaimed home of the "comic strip."

Part modern art project, part traditional chapel, this see-through church is only substantial from the right angle.

This narrow Ghent alleyway is an officially sanctioned street art tunnel.

This fairytale castle was built as a show of power but was nearly torn down thanks to its history of torture.

This monolithic slab of art metal crowns a German slag heap.

This massive steel pyramid can be entered by a set of jagged steps that let visitors stand in its heart.