It’s early in the morning but the sun is already sharp above us, the air clear. We’re among the first onto the grounds of Monte Albán that day, which I like, because the grassy plains between the pyramids stretch out empty before me. What I imagine as a once-busy square is now abandoned like the city itself was about a thousand years ago. We scale the pyramids, shoes disturbing tufts of grass that have made a home amid the rocks. We look out at the flattened ridge top before us. How did they do that? We walk around the edge of the city and look out over the valley below. How did they build a city, up here? And why did they leave? What…