What explains how coal deposits formed?

A coal deposit is a lenticular layer of carbon and carbon compounds formed from the remains of plants and basically ancient Geography explains how coal deposits are formed. To get a coal deposit you need the plants to be growing in swampy conditions so that as plants grow and then die, they fall into the the swamp waters to be replaced by new plants so that in time, a thick layer of dead plants builds up at the bottom the swamp (you need about 3 feet depth of plant matter to eventually form 1 foot of coal). You then need to rapidly bury the dead plants with sediments (sand and mud) before it rots away.

Thus we need a swampy place for plants to grow which, every so often, is subject to deposits of sediment. Today we call these places River Deltas.

Once the swamp gets buried by more and more sediments, the water gets squeezed out of the plants and over time, as the swamp get buried ever deeper, the heat from inside the earth changes the plants into coal.