This exclusive resort has completely covered a tiny Balkan island in a 5-star warren of guest rooms.

A tiny church on a man-made island in the Adriatic.

A curious cave church carved into the face of a cliff.

These historic defenses ensured the city within would survive hundreds of years of occupations, sieges, and invasions.

The feral felines of Roosevelt Island have a small army of allies and an abandoned hospital all to themselves.

An abandoned swimming pool decays on the outskirts of Berlin.

This abandoned subway station beneath the Waldorf-Astoria hotel is allegedly used to secretly transport presidents.

Amid the bustle and noise of the Financial District hides a secluded garden oasis above the city streets.

La Monte Young and Marian Zazeela's "Dream House" will immerse you in an ever changing world of sound and light.

A wunderkammer of mysterious objects and stories.

A surreal museum of extraordinary industrial objects.

Started as a kitchen garden, now the second largest botanical garden in the world.

The most visited, and possibly most controversial, museum in Germany.

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

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

Two awe-inspiring globes from the reign of the Sun King hang from the ceiling of France's national library.

A museum of carnival attractions where you are invited to ride the rides.

This castle-fortress is a living example of the transition from medieval defenses to modern fortifications.

This crumbling, graffiti-covered mansion sits tantalizingly within reach of the proletariat who frequent a public park.

Bits of architecture from a town destroyed in World War I now decorate this hotel garden.

Former hunting lodge of a duke who was an avid collector of impressionist works.

This abandoned-looking amusement park is frighteningly open for business.

For a brief time in 1960s France, families could climb into a tiny submarine and be pulled along an underwater journey.

This off-limits room in the National Library of France is home to a system of pneumatic tubes, once used to bring books to readers.