From their website:
This playground is located in the northwest quadrant of a 19.7-acre recreational facility built on the former site of a reservoir. On May 1, 1887 the City of New York acquired the property northeast of Bainbridge Avenue and East 207th Street, and two years later opened the Williamsbridge Reservoir. Measuring 925 feet long by 525 feet wide, and with a 46 foot high embankment, the oval receiving reservoir had a capacity of 120 million gallons of water. It was linked to the upstate reservoir at Kensico by a 48-inch diameter cast-iron pipeline extending 15.2 miles.