Formation of Coal
About 300 million years ago, the Earth had dense forests in low lying wetland areas. Natural processes like flooding, earthquakes, volcanoes, etc., buried these forests under the surface of the Earth.
As more soil deposited over them, they were compressed. The temperature also rose as they sank deeper and deeper. Under high pressure and temperature dead plants got slowly converted into coal.
This slow process of conversion of dead vegetation buried deep under the Earth into coal is called carbonisation.