Tuomas Sandholm remembers the year the cod disappeared from Helsinki. He was about 10 years old and spent much of his free time on or in the waters around Helsinki, Finland, learning to sail and fish. Typically, when the cod were running, Sandholm remembers barely putting his bait in the water before he got a bite. But that particular summer, there were no cod. They seemed to have vanished inexplicably. “I felt that very, very personally,” Sandholm says.