Coal and petroleum have been formed from biomass which has been subjected to various biological processes. Coal is the remains of tree, ferns, and other plants that lived millions of years ago. These were crushed into the earth, perhaps by earthquakes or volcanic eruptions. They were pressed down by layers of earth and rock. They slowly decayed into coal. Oil and gas are the remains of millions of tiny plants and animals that lived in the sea . When they died, their bodies sank to the sea bed and covered by silt. Bacteria attacked the dead remains, turning them into oil and gas under the high pressure they were being subjected to. Meanwhile, the silt was slowly compressed into rock. The oil and gas seeped into the porous parts of the rock, and got trapped like water in a sponge.