Here's how it forms:

1. Accumulation: Large amounts of plant material, often in swampy environments, accumulate over time.

2. Burial: These layers of plant matter are buried under sediment, increasing the pressure and heat.

3. Decomposition: Bacteria break down the plant matter, releasing gases and leaving behind a carbon-rich residue.

4. Transformation: Over millions of years, the intense pressure and heat transform the residue into different types of coal, depending on the degree of transformation:

* Peat: The least transformed form, soft and brown.

* Lignite: A slightly harder and darker form.

* Bituminous coal: A hard, black, and high-energy form.

* Anthracite: The most transformed, hard, black, and lustrous form.

Coal is a valuable energy source and is used to generate electricity and produce steel. It's also an important component of many industrial processes.