Hopefully they get to the bottom on occasion.
In the winter of 1977-78, when my wife was still a grad student at Indiana University, the coal miners went on strike, and by the time the winter was over the University reached the bottom of the coal pile used to heat the campus. It turned out that the coal at the bottom was several years old and had deteriorated to the point that it no longer generate much heat when burned.
(they ended up starting spring break a week early)