In the middle of downtown Brooklyn lies a 165 year old secret hidden right below the surface (closed to the public indefinitely).

This small, hidden cove is home to abandoned boats.

Empty, but not abandoned, this cavernous Brooklyn loading dock was once considered the largest individual building in the world.

A Victorian cemetery in the heart of Brooklyn remains a popular mecca of morbid curiosity.

This innocent-looking townhome is actually a subway exit.

Abandoned power station once home to a squatter community, now being developed into an arts center.

Just your average Marie Antoinette–themed speakeasy in an abandoned 19th-century subway tunnel.

A partially hidden wall graffitied by spectators of the 1976 Bicentennial, recently rediscovered.

NYC's legendary underground network for bovine transportation.

A veritable variety of odds and ends.

Visit the site of a subway powered by nothing more than air--and lots of it.

This little museum is built on a theater that was once a wildly elaborate speakeasy.

The one-time office of the architectural firm that ushered in the Gilded Age is now a Club Monaco.

This Upper East Side apartment co-op was designed as a tuberculosis sanitarium.

Currently housing a corporate bank branch this building at the southern tip of Manhattan was once the gateway to luxury vacationing.

Gang fights don't just happen on the streets, they happen under them too.

Surrounded by low rise buildings and partially converted into an auto body shop, this marble archway is the only vestige of a forgotten Manhattan estate.

Manhattan's Thirteenth Avenue was made on landfill then deliberately destroyed — apart from one small, obscure block.

A commemorative plaque hides a door that leads down to the New York Public Library's underground book bunker.

The greatest real estate coup of all time.

Red frames showcase remnants of one of New York's original subway stations.

Once considered the height of medical modernity, this structure is now the oldest remaining component of Roosevelt Hospital.

During the 19th century, you could pay for violence off a prix fixe menu on this Manhattan street.

A strangely angled West Village home is the only monument to an explosion that took the lives of three American revolutionaries.