An Italian hillside ghost town.

In the summer, the area around this sculpture transforms into a vibrant concert venue.

A statue honors the children who fought Nazis in Poland.

A tribute to the Polish insurgents who fought in the failed attempt to end their city's Nazi occupation.

After the famous composer died in Paris, his sister snuck his heart out of the country to honor his wish that it be buried in their native country.

The giant statue and its model solar system honor the native Pole who declared that the Earth revolves around the Sun.

Sweden's first nuclear reactor, R1, can be visited 25 meters underground.

Two planes stand abandoned after the closure of an aircraft-themed restaurant.

The house that took a plunge into the facade of a museum.

A line of abandoned, alien-like fortresses dot the mountains of North Italy.

A series of modernist treehouse hotel rooms grace the outskirts of an Arctic forest.

This real life asylum of horrors is now an abandoned ruin that hides the unbreakable code to one patient's insanity.

This quaint Italian town keeps losing ground as the sides of its plateau slowly erode.

Abandoned medieval hill town.

This truly palatial work of Communist-era architecture now sits crumbling on a Croatian beach.

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

An ethereal glowing blue ocean cave in Croatia.

The abandoned "Red Star Train Graveyard" is the final rusting place for precious artifacts of train history.

An ancient passage filled with stalls selling used books, once frequented by Vincent Van Gogh.

This gorgeous reading room houses the oldest and largest art history library in the Netherlands.

This 22-floor tower's Skydeck offers panoramic views of Amsterdam.

When the rain falls, this drain system turns into a musical instrument.

The site of a concentration camp where inmates were made to take part in one of the largest counterfeiting operations in history is now hauntingly empty.

A crumbling bakery where forced laborers from the nearby concentration camp toiled to feed their fellow prisoners and Nazi soldiers.