New Yorkers will proudly tell you that the city has some of the finest tap water that you can find. But for nearly a century, all of Manhattan has been served by a single water tunnel that hasn’t been shut down for maintenance since it first opened in 1917. That’s no longer the case. The city has finally completed work on a water tunnel designed to provide some redundancy to Manhattan’s lifeline. The tunnel runs 500 feet below the surface and is 12 feet in diameter.