On Monday, 20 April, for the first time on record, oil prices went negative. Futures contracts for May deliveries of West Texas intermediate grade crude oil changed hands for -$40 a barrel. In physical markets in the United States today, Oklahoma Sour and Wyoming Sweet are trading for -$5.75 and -$8.50 a barrel. Oil companies are extracting crude from the ground and paying people to take it away. The immediate cause is that oil storage facilities are almost full and oil, unlike bulk natural resources such as sand or gravel, cannot be piled up in fields. The storage tank farms in places like Cushing, Oklahoma will soon reach maximum capacity. The problem is not unique to the United States. The Covid-19 pandemic means global demand for oil has fallen by around 35 per cent. At the same time, a war is being waged using oil flows.