Coal is a fuel that is found in layers under the ground. These layers are called ‘coal mines’. Coal is known as a fossil fuel because it was made millions of years ago from dead plants. Coal is used for heating and in making electricity, gas and chemicals. It is also made into another fuel called coke. It is the most abundant fossil fuel in the world and meets 30% of the world’s energy demands. Formation of coal began some 250 million years ago, in an age called ‘carboniferous period.’ During that period our Earth had many swamp-like seas. Because of the hot and moist climate, gigantic plants were able to grow. When these plants died, they sank into the swamp and new plants replaced them. As the newer plants died, layers of dead material began to form. Mud and sand settled on top of this layer and pressed some of the water out. Eventually, the land rose and more plants grew on the surface. These new plants died, decayed and were covered by sediments again. This cycle continued for millions of years. Many layers of stratified matter formed, as decaying material was forced further beneath the surface. These layers produced great pressure. The pressure together with chemical reactions gradually, transformed the decayed material into coal. In this way, coal was formed in many layers inside the Earth. Today, we have to dig to great depths to obtain it. This explanation of coal formation is based on the fact that the clear impressions of ferns are found within the coal. Sometimes, patterns of bark have also been observed which again proves that it is formed out of the remains of trees, plants and shrubs millions of years ago. Coal consists mainly of carbon, hydrogen, oxygen and nitrogen. There are four types of coal — peat, lignite, bituminous and anthracite. Peat is the material produced in the first stages of coal formation. When removed from the ground, it may contain 90% of water. Dry peat contains up to 60% carbon. Lignite contains 60%–75% carbon. Bituminous contains upto 80% carbon. Anthracite is the final product of the coal formation process and contains about 95% carbon. Obtaining coal from the mines is a difficult job. First, the dirt above the coal deposit is removed. When the coal is exposed, explosives are used to break it into smaller pieces. The coal thus collected is loaded into wagons and lifted to the surface. The miners enter and leave the mine by an elevator through a vertical space called the shaft. Coal mines can easily catch fire and it is very difficult to bring the fire under control. Hence, that section of the mine is isolated from the rest to prevent the fire from spreading. The leading coal producing countries are the United States of America, Russia, China, Germany, Poland and Great Britain. In India, coal mines exist in Andhra Pradesh, Bihar, West Bengal, Rajasthan, Maharashtra, Orissa, Assam, Jammu and Kashmir and Madhya Pradesh. About three billion tons of coal is mined every year in the whole world. India ranks third in the world and accounts for about 100 million tonnes in 2006.