there was a long period of time -- 300 million years -- when trees fell over but didn't rot. microbes that could ingest wood hadn't evolved yet.

so the earth was just littered with trees! those trees got packed down by other trees that fell on top of them, and that became coal

this was during the "Carboniferous" era

Carboniferous literally means: coal-bearing

(in latin, carbō="coal" and ferō="I bear, I carry")

my original sources were conversations with science friends followed by reading this article https://www.nationalgeographic.com/science/phenomena/2016/01/07/the-fantastically-strange-origin-of-most-coal-on-earth/

HOWEVER, i now think wood not degrading is an outdated theory. there's some pretty convincing evidence against it -- for example, absence of lignin decay for millennia should mean we have a lot more coal than we actually do

https://www.pnas.org/content/113/9/2442