"Inside The Lost Jehovah's Witnesses Tunnels Under Brooklyn Heights"

Gothamist:

When the Jehovah's Witnesses moved out of Brooklyn Heights over the past few years, cashing in on their now-valuable properties and moving upstate, they left behind a collection of buildings that had been their headquarters for over a century.

...

Out of sight of most New Yorkers, though, was the development underground: a series of tunnels that ran beneath the streets to connect a number of the JW properties.

...

housed offices, a grocery store, laundromats, and other services.

And more New York underground:

Someone—no one really knows who—has been disrupting the train system, sneaking into cabs, pulling the emergency brakes, and grinding not just one train, but entire lines in the system to a halt during the busiest hours of the day.