Collapsing castle on the Hudson river was once the storage site of military surplus business.

A self-funded cemetery with a long list of inhabitants.

A museum in the childhood house of a medical pioneer.

Budapest's Communist-era statues, banished to the suburbs.

An underground city of salt.

Giant WWII fortresses that fired 8,000 rounds a minute, now home to thousands of pigeons.

A spherical "micro-nation" in the heart of Vienna.

Try not to get slimed in Tribeca.

The one-stop shop for all of your superhero (or evil arch-nemesis) needs.

In all of the 29 years it was in operation, no prisoners ever escaped successfully... or so they claim.

The first American department store, and home to the largest operating musical instrument in the world.

In the early 1800s, surgery was also a show.

World's first "penitentiary," meant to be humane, drove men insane.

America's most famous museum of medical oddities is home to the remains of Albert Einstein's brain.

One of Australia's largest secondhand bookstores.

Mysterious 500-year-old tapestries depict a unicorn hunt.

A Natural History and Science Emporium.

A self-made museum in Queens, New York continues to display the work of a sculptor known for paving his own way.

This mosaic dedicated to John Lennon was tended for years by a Beatles super-fan.

A crumbling hospital from the 1850s on Roosevelt Island.

A graffiti Mecca welcoming artists from around the world.

Nature and beauty amidst pollution and industrial wreckage.

A drawbridge named for a potentially intersex Polish national who fought alongside George Washington during the American Revolution, and an example of "Pulaski Red".

The factory for the popular Dumbo location, the Greenpoint shop is a quiet gem.