If you examine aerial images of southeastern Queens, you'll find a number of these mysterious metallic disks lurking about. My best guess is that they are actually in-ground storage tanks for the old groundwater supply system, but I don't know for sure. Anybody have a more definite answer?
UPDATE: I'm now confident saying this is an old in-ground water storage tank. The property was formerly owned by the Jamaica Water Supply Company (JWS provided the area with well water before selling its Queens properties to the city) and now belongs to the Department of Environmental Protection (the department in charge of the city's water supply). And it looked the same in 1996, the year the city bought the JWS properties, as it does today.