A crumbling World War I fort that is reached through a series of concrete tunnels.

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

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

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

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

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

A natural gas leak provides a flickering flame beneath a waterfall.

This Hungarian castle was built out of cardboard and wood until it proved so popular that it got upgraded to stone.

This scalding hot river was thought to be a myth until one geoscientist made it his quest to study the mystical waters.

The bunker where Hitler took his own life is hidden under this deliberately ordinary parking lot.

A whimsical and twisted animatronic monster tour beneath the streets of Berlin.