Coal is a non-renewable source of energy, meaning the earth will eventually run out of it… and once it has gone it has gone. It takes thousands of years for coal to form naturally so in realistic terms we cannot make anymore.
The energy in coal comes from HUGE dead plants that once inhabited this earth around 300,000,000 years ago (way before the dinosaurs). Once these goliath plants and ferns died they formed in layers at the bottom of the swamps they grew in.
Over time, water and dirt started to gather and build up on top of the plant matter… Over thousands and thousands of years under intense heat and pressure the matter would undergo chemical and physical changes pushing out all of the oxygen leaving what we know as coal.
Below is a little graphic showing the process of coal creation.