Back in the day, when charcoal was needed for the iron furnaces, they would cut down huge amounts of wood and dig a giant pit. Pile the wood in the pit and start it burning. Then they would cover the top of the fire with earth to restrict the air. Small holes would be left to allow some oxygen in, but it was mostly left to smolder for several days. Once it quit smoking, etc it would be dug back up as charcoal.
I remember this from a tour of the Cacoctin Mountain Iron Furnace when I was a kid. A little googling might lead to some more info.
-d