How was bituminous coal formed?

There is a generally accepted hypothesis as to how the coal was formed. Approximately 300-400 million years ago, huge impenetrable forests grew on Earth, consisting mainly of giant-sized ferns and their relatives. Naturally, the plants died from time to time, decayed and fresh layers were layered on the layers of semi-decomposed plants for millions of years. Thus, dead plants in an airless environment with the help of microorganisms turned into peat, and then millions of years later – into coal.