ASTRONAUTS LOOKING DOWN at Earth from space have gazed at the planet with a sense of awe and wonder. When American astronaut Edgar Mitchell saw his home planet from thousands of miles away, he described it as "a sparkling blue and white jewel." His observation aptly conveys why Earth is often referred to as the Blue Planet. Even though there are more than 50 million square miles of land, a much greater area is taken up by oceans, which cover nearly three-quarters of the planet's surface. These vast bodies of water are a valuable resource because of the role they play in regulating climate as well as providing a habitat for the most plentiful and diverse creatures on Earth.

Life beneath the sea

Although the exact number of ocean species is not known, about a quarter of a million have been discovered. According...