The first description of mare sporco, or “dirty sea,” in Italian dates back to 1729. But in the early 2000s, marine mucilage started breaking out pretty much every year, which scientists, in a 2009 paper, linked to climate change. (Huge swaths of marine mucilage have also turned up near Turkey at least once before, in 2007.) You might think of the snot as a symptom of “ocean flu,” says Antonio Pusceddu, a marine ecologist at the University of Cagliari, in Italy, who co-authored that paper: The snot’s appearance is a sign of deeper sickness in the sea, caused by climate...