Some of the ocean's most skilled divers are marine mammals. Sperm whales and elephant seals can stay underwater for almost two hours. Sperm whales have been recorded at depths of more than 9,000 feet and elephant seals at depths greater than 3,000 feet.
Hearing is the most important sense to many marine mammals.
In fact, most whales and dolphins rely on clicks, whistles or songs to communicate with each other. Two of the world's most endangered species of marine mammals live in U.S. waters. There are as few as 300 northern right whales remaining off the U.S. East Coast, and only 1,300 Hawaiian monk seals live and breed at a few locations in the Hawaiian Islands. In 1994, the gray whale became the only marine mammal species to be removed from the Endangered Species Act list. Almost driven to extinction by commercial whaling in the late 1800's, the gray whale's recovery is a true environmental success story.