How does coal form in the ground?

Answer 1

From the pressure of overlying sediments squeezing the water out of ancient plant remains.

Answer 2

Coal forms from the remains of ancient plants that have been buried and subjected to high pressure and heat over millions of years. As the plant material decomposes and undergoes chemical changes, it transforms into peat, then lignite, sub-bituminous coal, bituminous coal, and finally anthracite coal, with each stage containing less moisture and more carbon.