Blue whales are not just the world’s largest mammals. They are the largest animals ever to exist on our planet, longer than two city buses back-to-back. These gentle giants have been extinct from the Atlantic coast of Spain for more than four decades, but they’re now coming back. Researchers are wondering why. Bruno Díaz heads the Bottlenose Dolphin Research Institute in Galicia, Spain. He joins host Marco Werman to discuss the possible reasons for the whales’ return. Hint: it’s all about the krill.