Coal formed millions of years ago, around that time, the earth was covered in huge swampy forests. As plants, such as ferns died, they sank to the bottom of these swampy areas. The environment in which the dead plants ended up, prevented decay. Over time, more material built up and the pressure and temperature increased. This increase in temperature and pressure over time, converted the dead plant matter into the coal that we (unfortunately) utilize as a source of energy today.