So, how'd the coal get so deep ?
Good question.
Well, new soil is constantly being laid down over the old do to volcanic eruptions, earth movement tectonic activities, etc.
While this doesn't amount to much over a year's time frame -over a period of thousands of years it adds up - even over a few hundreds of years. Look at ancient cities, Roman ruins - they are all below curren ground levels.
Coal deposits were laid down millions of years ago.
That has always been a more fascinating mystery to me than the Biblical Flood.
Here you have a layer of coal with basalt rock (cooled molten lava) on top of it, sealed up for the ages. Not only that, there may be another layer underneath the first one.
Had the hapless forest (or other plant life) been left on the surface of the ground, it would have slowly rotted, oxidized and gone away, never to be seen again.
Those layers of coal could tell us much more about the pre-history of the earth if we were interested enough to study them.