At the mouth of the Mississippi River, there is a dead zone. A hypoxic zone: water so depleted of oxygen that almost nothing can survive in it. It forms every summer. At its largest, measured in 2017, it covered approximately 8,776 square miles. An area larger than New Jersey, sitting in the Gulf of Mexico, producing almost no marine life. The cause: nitrogen runoff from the Corn Belt. The 90 million acres of corn in the American Midwest require synthetic nitrogen fertiliser, produced from natural gas, applied annually. The soil cannot hold all of it. The excess runs off into streams, into the Missouri, into the Mississippi, and down to the Gulf, where it fertilises algae blooms that consume the oxygen when they decompose. The dead zone has existed in some form since the 1970s, when nitrogen fertiliser use on the Corn Belt expanded to the scale it is now at. The shrimping industry in the Gulf of Mexico has spent fifty years working around a seasonal dead zone the size of a small state, caused by the runoff from a crop that is 40% destined to become ethanol. The ethanol is a fuel additive. The dead zone is an externality. The externality is 8,776 square miles.