Everybody wants Chaco to be a mystery. Stuart, an anthropologist at New Mexico who’s spent his career out there, knows it isn’t. It was a boom town. Great houses five stories high, dead-straight roads running for miles across nothing, macaws and turquoise coming up the trade lines, timber hauled in from mountains a hundred miles off. Somebody was running a hell of an operation. His frame is simple. Societies go one of two ways: Powerful ones grow fast and spend everything to do it, and efficient ones grow slow and hang on. Chaco went powerful. Then the rain quit in the 1130s and the whole thing folded inside a generation. The part Stuart cares about is who was standing when the dust settled. Farmers who scattered out into little villages, planted a bit of this and a bit of that, and shared what came in. Those are the Pueblo people today, still there. Then he points the same question at modern New Mexico, and you can see where that’s going.