circular tunnel, part of the system that supplies water to New York City from the Delaware River near its source and from other streams in the Catskill Mountains. Running deep in bedrock for its original length of 85 miles (137 km) from Rondout Reservoir in the Catskills to the Hill View Reservoir in southern Westchester county, the aqueduct has been extended to a total distance of 105 miles (170 km). The aqueduct was built between 1937 and 1953, with extensions continuing until 1965. It runs in some places to a depth of about 2,500 feet (750 m) below the surface and reaches a maximum diameter of 19.5 feet (6 m) in its last 13 miles (21 km).