From rough seas to crystal clear waters of the Florida springs, David Attenborough swims with sea otters and dives with manatees, as he follows those mammals that left dry land millions of years ago and returned to the water to feed. He sails in the Pacific Ocean to find the largest mammal that has ever lived on this planet, the blue whale, a hundred feet or thirty meters long. He also witnesses a pod of high-speed dolphins pursuing their fish dinner....