Wikipedia: The Shandaken Tunnel is an underground tunnel in Eastern New York State, part of the New York City water supply system. It was constructed between 1917 and 1924. The aqueduct starts in Gilboa, New York at the Schoharie Reservoir, which is in the counties of Schoharie, Delaware, and Greene. The water flows south towards the Esopus Creek in Ulster County. It finally empties into a man-made pool in Allaben, New York, within the Town of Shandaken, and enters Esopus Creek there. The tunnel is over 18 miles in length, and carries water through Greene County and Ulster County. Water flows through the tunnel by gravity; it slopes at a rate of 4.4 feet per mile . It is 11.5 feet high, and 10.2 feet (3.1 m) wide. There are seven shafts along the tunnel, serving as air vents to introduce oxygen to the water, and to bring out impurities.