You often see them from the window of a cross-country jet: huge, perfect circles in varying shades of green, gold, or brown laid out in a vast checkerboard stretching to the horizon. Across much of the American Midwest and on farmland throughout the world, these genuine crop circles are the sure sign of an automated irrigation system—and an emblem of a revolution in agriculture, the most ancient of human occupations. At the heart of this transformation is a single concept: mechanization.