Spy Pond is one body of water in a chain of ponds located on the eastern side of Duston Heights.
Byron "Fergie" Ferguson and Johnny Dixon are hiking along the trails that surround the chain of ponds when they come to the top of a hill overlooking this pond and, "on the far bank", the "old brick pumping station, with its green copper turrets. Its windows had turned to blobs of golden fire" (The Revenge of the Wizard's Ghost, 37).
InspirationHaverhill has a number of ponds on its eastern side near Winnekenni Park that were within walking distance of Bellairs’s Hamilton Avenue home.
There is, however, no pond in Haverhill previously or currently named Spy. The closest is Spy Pond in Arlington, Massachusetts. Bellairs may have visited the Arlington area or heard the mysterious-sounding name and elected to use it in his writing.
Bellairs may have had Haverhill's Great Pond, also known as Kenoza Lake, in mind since "the old brick pumping station, with its green copper turrets” seems to refer to the city’s former waterworks building on Kenoza Lake’s northern shore.