Ruins of a German Castle on top of a towering stone pillar.

A tour of underground Berlin takes you through WWII air raid shelters and tunnels used to smuggle people out of East Germany.

This futuristic German spa allows visitors to float in a pool of salt water while zoning out to underwater techno.

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

An eerie, derelict amusement park in Berlin.

Built as an anti-fascist tribute, this surreal Yugoslav memorial became an unwelcome symbol after the Croatian war for independence.

A tribute to an iconic act of defiance from the Croatian War of Independence involving a small car.

Croatia's "hairy stone" fortress is one of the most well preserved Istrian castles.

The now-overgrown beach resort was once frequented by Yugoslav military elite.

Formerly Yugoslavia's "forbidden island," this sleepy little city still holds the abandoned remains of military strongholds.

When Croatia became independent of Yugoslavia, the splintering governments left behind these concrete tunnels.

Croatia's towering concrete blossom grows from the site of a World War II concentration camp.

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

This former estate of a fascist leader is now a burnt out ruin leading to a system of secret tunnels.

Longest fortress system anywhere in Europe.

This scale model of our solar system hid in plain sight for years before a physics team sought out all the planets.

Once a state-of-the-art tuberculosis treatment spa, these ruins are now ruled by paintball players.

This huge metal monument is slowly being stripped to its concrete skeleton.

A musical instrument played by the sea.

Colorful concentric circles honor a small Croatian town's supposed spot at the nucleus of it all.

Humble beginnings for one of the world's most famous scientists.

With its mine closed, this old village is starting to resemble a ghost town.

A forgotten and crumbling Yugoslav monument to the victims of fascism.

Yugoslavia's biggest underground airport lies in ruins.