water and gravity have formed the surface to have few local minima From the article: "It turns out that the earth is filled with local minima" (I suspect the

water and gravity have formed the surface to have few local minima From the article: "It turns out that the earth is filled with local minima" (I suspect the deal is that you're right that water always flows downhill to the ocean; but that it often happens at scales too small to capture in the GIS data, or underground.)