4. THE SEA

The Earth, our invaluable planet, which is mostly covered by water. The great basins between the continents, in which all this water lies, are actually more varied than the surface of the land. The highest mountain on earth, Everest, would fit into the deepest part of the ocean with its peak a kilometre beneath the surface. On the other hand, the biggest mountains in the sea are so huge that they rise above the surface of the waters to form chains of islands.

The sea was first formed when the earth began to cool soon after its birth and hot water condensed on its surface. This early water was not pure, like rainwater, but contained significant quantities of chlorine, iodine and nitrogen, as well as other rare substances. Since then, other ingredients have been added. As continental rocks weather and erode, they produce salts which are carried down to the sea by rivers. So, over millions of years, the sea has been getting saltier and saltier.

Life first appeared in this chemically rich water some 3,500 million years ago. We know from fossils that the first organisms were simple, single –celled bacteria and algae. Organisms very like them still exist in the sea today and are basis of all marine life. The biggest of them is about a millimeter across, the smallest about one-fiftieth of that.

They exist in immense numbers-a cubic meter of sea water may contain 200,000 –and they drift in the water. These minute organisms are able to harness the energy of the sun to build the molecules which form their tissues. Among them float vast numbers of small animals which feed on them and which together are known simply as ‘the plankton’, a living soup which is the main diet of other bigger creatures.

A. Match the words in Column A with Column B.