My favorite subscription in the combined Dowdey arsenal of periodicals is far and away National Geographic. September’s issue featured an especially striking cover — the kind that made me flip right to the feature article. It’s a split image of Manhattan, one side grey with the familiar skyline, the other green as any unexplored wilderness.

And this wasn’t mere guesswork. Someone actually dug deep enough below the borough’s foundations to discover landscapes of swamps, beavers and old-growth hickories.

Ten years ago, Eric Sanderson, an ecologist at the Wildlife Conservation Society, bought a coffee table book of old New York maps. He was struck especially by the “British Headquarters Map,” a detailed record of the city’s streams, swamps, orchards and farmhouses from 1782 or 1783. After realizing some of the map’s features still existed, he decided to see how well the locations meshed with the city’s current layout.