Comment by WalterBright

Correct, but burying trees today isn't going to turn them into coal.

The big difference is that when the current coal layers were formed, bacteria to decompose trees hadn't evolved yet. There was a huge gap between trees forming and the ecosystem to break down trees forming, which led to a lot of trees dying and nothing being able to clean it up, which meant it was just left lying there until it was buried by soil and eventually turned into coal.

Try to bury a tree today, and nature will rapidly break it down. It won't form coal because there's nothing left to form coal.