The world's largest animal is in trouble. Blue whales were nearly hunted to extinction in the early 20th century, and by the time hunting was deemed illegal in 1966, only a few thousand remained, down from a prewhaling population of about 240,000. Today, the population has rebounded to between 10,000 and 25,000 individuals. They remain are under attack from seas polluted by chemicals such as PCBs and noise from boats and sonar equipment. Add the warming and acidification of the seas, and the outlook is bleak.