From YouTube:

New York City is the biggest city in the country, but it's got some dark secrets. Founded by covert groups, overrun with gangs and mob bosses, and ruled by secret societies, this is the true foundation of the city that never sleeps.

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Some of the cool things you will see from Time Out NY:

Fake subway exteriors

If you take a stroll through Brooklyn Heights, be sure to pause in front of 58 Joralemon Street, near Willow Place. It appears to be a normal brownstone, but after a more careful look at the three-story building, you’ll notice blacked-out windows. It’s actually an emergency subway exit and ventilator, situated along the 4 and 5 train tunnels. Peer through the cracks of the front door, and you’ll see what looks like a subway platform. A similar substation exists behind the walls of 108 East 19th Street, near Irving Place; it can be found behind a large set of garage doors.

Cow tunnels on the West Side

Long before the neighborhood became a tourist trap, the Meatpacking District was the aptly named home of many Manhattan slaughterhouses and packing plants. At the turn of the 20th century, cattle came from New Jersey to the Manhattan docks via barge. To simplify their transport from dock to slaughterhouse, the city built underground cattle tunnels, helping the animals avoid everyday human traffic. The passes are located at 34th Street between Eleventh and Twelfth Avenues, and along 39th Street at Twelfth Avenue; they were rediscovered during an archaeological documentary study conducted for the 7 train extension in 2004.

Man, I would love to get a tour of these. The NY Public Library has underground values, and there are crypts in Little Italy.