Earlier this week I visited Machu Picchu in Peru, which features extensive terracing for crop-growing. Five-hundred years ago the Incans took full advantage of a uniquely temperate microclimate on sites like this along the border of the Amazon. First of all they engineered a drainage system out of rocks brought up from the river below. Then they covered it with dirt laboriously hauled from fertile plains at lower elevations. Next they evidently experimented on different crops at different levels to get the best interaction with varying temperatures each step of the way down from the peak. According to a team of agronomists and archeologists from U Penn who reproduced the Incan farming conditions, yields of potatoes came in at two or even four times what would be expected. All this is quite impressive—building a self-sustaining city at nearly 8000 feet on a peak with only a few, small flat areas.