The complex has been abandoned since the Croatian War of Independence, barring a stint as a set for 'Game of Thrones.'

The remains of a sunken military ship wreck visible through clear water.

Yugoslavia's biggest underground airport lies in ruins.

Large, airy hallways hide within an impenetrable fortress that beckons urban explorers.

A wonderfully strange collection of 500+ stuffed frogs doing people things.

This stunning, out-of-the-way Croatian monument commemorates a community's uprising against occupying fascist forces.

A musical instrument played by the sea.

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

An ethereal glowing blue ocean cave in Croatia.

The history of occult science is turned into a creepshow at this lurid Prague tourist attraction.

A Jenga-style cylindrical tower of books that appears to go on forever.

Two stunning 17th-century libraries and a cabinet of curiosities.

A church of bones, decorated with 40,000 human skeletons.