Human sewage is the top source of eutrophication in the developing world, but in the U.S., Europe and China, animal waste and fertilizer are the main culprits. Every year in the U.S. alone, some 10 billion chickens, 80 million cows and 149 million pigs produce 500 million tons of manure, much of which is sprayed on fields. Farmers also add another 55 million tons of synthetic fertilizer to their cropland, much of which washes back into the watershed. The result: The largest dead zone on Earth spreads from the mouth of the Mississippi. In the summer, when Midwestern agricultural runoff reaches its peak, it can grow to be the size of New Jersey.