Almost all coal that is used today has its origins in the Carboniferous period. The Carboniferous period was a warm and wet one, with the majority of the land being swampy and covered with dense rainforests of ancient tree species. By the end of this period, the climate quickly changed, resulting int he collapse of the rainforests. As the trees were dying out, they were falling in the swamps, quickly being covered by the mud, so remaining largely preserved. Over time they got exposed to higher pressure and temperatures as they were getting deeper into the crust, eventually resulting in the formation of the coal.
There are so many examples for that in different areas, like TPT1 experiment carried out in our lab recently.Here's one link: http://www.alfa-chemistry.com/tpt1-cas-167218-46-4-item-290583.htm
