The abandoned husk of a mental institution that failed to save a Jazz Age icon still sits nearly unchanged.

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

Reforestation in action is the main attraction in this abandoned quarry.

This abandoned ironworks was once the most important military supplier in America.

The famous author's cottage hideaway.

The home of one of America's most wealthy industrialists is now a historic site that remembers a more decadent time in the country's history.

An Escher-inspired library dedicated to the wonders of the human mind, yet closed to the public.

This castle-like building in Manhattan helped destigmatize cancer in the 19th century.

Manhattan's deadly West Side railroad, which killed hundreds of New Yorkers, is remembered by this simple plaque.

A simple midtown bar was once the home of a secretive and exclusive club staffed by "bunnies" under Hefner's strict rules.

This humble historic facade hides a history of boozing and the gentlemanly sport of rat-fighting.

Set sail on a small scale.

Site of the first pathological and bacteriological research facility in the United States.

The notorious headquarters of a corrupt political machine.

There’s a secret bunker inside the Brooklyn Bridge, but the city won’t say where.

This was one of the first (and last) university-built and operated nuclear reactors.

An odd, ancient window that may have once been part of a brutal prison is embedded in the side of New York's Police Plaza.

A posh members-only club with a literally dramatic history.

This abandoned subway station can still be seen on regular commuter trips.

This hidden library of physiological ephemera contains centuries worth of knowledge on the vim and viscera of the human body.

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

Tucked away amidst some of the most famous addresses in the world is New York's only fraction of a street.

Manhattan's lighthouse, erected at the insistence of the unsinkable Molly Brown.

Cryptic messages embedded in the New York City sidewalk.