The three major oceans of the world are the Pacific, the Atlantic and the Indian Ocean. Great ocean currents swirl around the earth, many of them thousands of kilometres long. Some are warm currents, some are cold. These currents have an enormous effect on the world's weather systems. Oceans also have layers of water at different temperatures.
Oceans support the greatest variety of life on earth, from microscopic plankton to giant whales.
The deepest parts of the oceans have barely begun to be explored, and new life forms are being discovered every year by deep ocean submersible machines. The floor of the ocean is called the benthic habitat while the water itself is called the pelagic habitat. Both support a diversity of animal life. Coral reefs, which grow in warm tropical and subtropical seas, are perhaps the richest marine habitat in terms of the diversity of life they shelter. The oceanic food chain begins with microscopic drifting plants called phytoplankton.