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Section C
Earth — a Living Planet
Earth looks like a big blue marble（玻璃球）; from high above the Earth and from the moon, the planet gleams and shines. The blue water in the oceans and seas of the Earth makes a dramatic image. The white clouds above the Earth add beauty to the picture. Water is the source of this beauty and the source of life on Earth. It is the reason people can live on this planet. Water is everywhere. It is in the air that people breathe. It is in the soil, the ground that grows the food. Water is in rock deep under the ground, in natural holding areas — in storage. In a real sense, water keeps Earth alive.
Nature has an unchanging amount of water. Nature has a perfect system for recycling water. Water is used again and again. It falls as rain. Then it goes to one of three places. It might sink slowly through the soil into the natural holding areas in the rock. It might disappear into the air quickly — by becoming vapor, or gas. It might run off into streams, rivers and oceans. By itself, nature can keep the balance and provide plenty of clean water for us. Nature recycles water.
However, people cause problems for this natural recycling system. Nature's recycling system can work well only if people work with the system and not against it. Some ways that people upset nature are easy to understand. For example, dirty sewage （污水沟系统） water from homes and factories must not mix with drinking water. People get sick from drinking contaminated water. Sometimes water from factories goes into streams and rivers. It enters into the groundwater. It can flow into lakes too. This kind of contamination from industry (waste water from factories) can be dangerous for people. If water contains poisons and chemicals, it is poison. Poison makes people sick; some poisons kill people as well as birds and animals. Without knowing, people can upset nature's recycling system.
Lakes and rivers add beauty to the world. People enjoy water for entertainment purposes, too. People enjoy swimming and playing in the cool water of a lake in the summer. They like to ride on boats on rivers. Many people enjoy catching fish in the rivers. They fish for food and for sport. However, in some places, the water of the lakes and rivers is no longer safe. These rivers and lakes are contaminated. The fish are dying because of the chemicals from farms and factories. People cannot swim in the polluted water.
There are other ways that people disturb nature. Some of these ways are not easy to understand. For example, of what use is a wetland? No one can plant crops on it. No one can build a house on it. Therefore, engineers have removed the water from some wetlands in order to make useful land. Many shopping centers stand on dry land today, land that was once wet and full of marshes. Yet the soft and wet ground of a marsh serves an important purpose in nature. In a marsh, the surface water can sink slowly down through the soil into the rock below. Nature's holding area fills slowly with that clean, filtered water.
Housing developments and shopping centers cover much of the Earth with paved and concrete surfaces. Water cannot sink through these hard surfaces. Rainwater cannot sink into the ground because of the buildings, roads, and parking lots. So it floods parking lots and flows into basements. Engineers build huge storm pipes or sewers （污水沟） to carry the storm water away, but these cause another problem. They carry all of the water away. Not much water can sink into the holding areas under the ground. The once unlimited supply of fresh, clean water is now limited. Nature's recycling system is in danger.
Because of water, Earth is a living planet. People can live here because of water. They build large dams to store water in huge man-made lakes. The water in these lakes can water farmland and provide water for cities. Water from the dams can make electricity, called hydroelectricity since it is made from water power. These hydroelectric （水电的） projects produce electricity for the people of nearby cities and towns. However, these dams also cause problems. In some places, the holding areas behind the dams have destroyed the environment for animals, birds, and plants.
One thing is certain — the balance of nature on this water planet is easily upset. And upsetting the natural water cycle （循环） on Earth makes significant problems for its inhabitants. All of us share these problems. Water gives life to our planet. We must learn to live in balance with nature, or our shining planet Earth will die.
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