Amid an unprecedented summer of extreme heatwaves, drought, and wildfires, several of Europe’s major rivers are drying up. Among them is the Danube, where record-low water levels are exposing long-sunken historical treasures and forcing officials to take drastic action to mitigate impacts to infrastructure and the economy. Water levels have fallen so low that dozens of World War II battle ships are breaching the surface. Last week in Bulgaria, the bones of a woolly mammoth turned up. And on Monday, the Romanian navy blew up a rock outcrop to help redirect cooling water to the nation’s only working nuclear reactor....