The remains of dead plants form deposits of coal. It is our oldest source of energy. Coal is often buried deep in the Earth. How did it get there?

280 million years ago was the Coal Age of Earth's history. Characterised by high temperatures and luxuriant plant growth. During this time large amounts of tropical plants, particularly ferns, died and rotted. Slowly at the bottom of marshes, they were covered by layers of mud and silt. Because of the weight, heat increased and as there was no oxygen, all combustion was impossible. The remains were compressed and eventually mineralized.... and so coal was created!
This process was very slow. The coal stagnated under the soil, buried in extensive veins between the layers of other, non-combustible rock. The layers of coal aren't always of the same thickness.