Although the sea covers more than two thirds of the earth's surface, we know less about its deep terrain than we do about the dark side of the moon. But however unfamiliar the underwater habitat might be, the ocean is closely linked to our lives. Though we cannot breathe below water, about 70 percent of the oxygen the planet needs comes from its oceans. At the same time, they absorb a great deal of the carbon dioxide we emit into the atmosphere, and that is making them increasingly acidic and threatening their ecosystems.

Among the countless animals that populate our oceans are those that also live on land. Sea lions and penguins are examples of such species, and both hunt in the water but rest and even breed on shore, where they have no natural predators. Other species have adapted to living on the border between oceans and land. Mangroves, for instance, grow on tropical coasts but their long roots are embedded in salty sea water. As such, they act as a natural protective barrier against coastal erosion and provide a unique habitat for numerous fish and other marine species.

Overfishing not only poses a risk to species stocks, but to entire ecosystems, and consequently represents one of the biggest threats to the world's oceans. Environmental groups are trying to create national parks in special marine regions, the idea being that underwater tourism helps both fish and fishermen by offering a whole new source of income.