Overview: Stoneman and his crew conduct a detailed study of two types of marine mammals that appear to be at a midway point in their total adaption to the sea: the walking seal, in the form of the sea lion and the crawling seal, as shown by the harp seal. This demanding expedition takes the diving team from the tropical warmth of Mexico to the icy cold of the east coast of Canada, here to dive beneath the ice with these highly specialized animals.