Because the world’s most important medium of transport is the sea, it is important to realise that the sea extends to many different areas, has different characteristics, and is home to a vast array of plants and animals. The surface of the sea changes according to wind strength and currents. The ocean bed varies from mountainous structures, deep valleys, coral reefs rich with life, and seemingly barren stretches. The same seabed has diamonds in some areas, and beneath the seabed lie vast oilfields and gas pockets.