The Danube, Europe’s second-longest waterway, could be threatened by a toxic red sludge that has entered one of its tributaries. This week, a reservoir for toxic sludge at an aluminum plant in Hungary burst, releasing 1 million cubic meters (more than 35 million cubic feet) of sludge into two neighboring towns. So far, four people have been killed, three are missing, and 120 have been treated for chemical burns.