Old forests on Earth,entered beneath the surface due to various causes like earth quake,land slide,great flood etc.staying under the surface for long no. of years due to temp.& pressure these woods converts to rock like state.These wood fossils are called Coal.

Coal is called a fossil fuel because it was formed from the remains of vegetation that grew as long as 400 million years ago. It is often referred to as "buried sunshine," because the plants which formed coal captured energy from the sun through photosynthesis to create the compounds that make up plant tissues. The most important element in the plant material is carbon, which gives coal most of its energy. Most of our coal was formed about 300 million years ago, when much of the earth was covered by steamy swamps. As plants and trees died, their remains sank to the bottom of the swampy areas, accumulating layer upon layer and eventually forming a soggy, dense material called peat. Over long periods of time, the makeup of the earth's surface changed, and seas and great rivers caused deposits of sand, clay and other mineral matter to accumulate, burying the peat. Sandstone and other sedimentary rocks were formed, and the pressure caused by their weight squeezed water f ...more