Coal is a type of fossil fuel that was formed from the remains of plants that lived millions of years ago. During the Carboniferous period (300 to 360 million years ago), much of the Earth was covered in lush swamps and forests. When these plants died, they fell into the water and were covered in layers of mud and sand. Over time, the intense heat and pressure from the Earth's crust combined with the lack of oxygen to transform these plants into coal.