More than 300 million years ago, plant life growing in swamps began to decompose. Layers of sand and mud covered the decomposing plants, and the decaying plants were compressed by the combined weight of water and sediment. Over time, the plant matter hardened and became coal. Coal deposits are mined all over the world, and coal was one of the major fuel sources in the world through the 1970s. In many countries, concern over the environmental effects of the burning of coal has led to a decline in its use. The smoke produced by burning coal has a high acid content, and creates an environmental condition known as "acid rain," which is harmful to plant and animal life. Despite the environmental concerns, some of the largest coal mines in the U.S. each still produce more than 450,000 metric tons annually, making coal mining a very profitable industry.