In underground mining, particularly coal mining, the gob is the space which has already been mined and is filled with waste or collapsed roof rock (i.e., in longwall operations). Sometimes, spontaneous combustion can occur in the gob, as this space is inaccessible and difficult to monitor.

Figure: The gob shown for an underground longwall mining operation.

In surface coal mining, the term gob is sometime used to refer to waste (usually tailings) piles that contain some coal which was not recovered during processing. In this case, the material is called a gob pile. Gob piles that were created before advanced processing techniques were developed are now subject to re-mining; which means that the material can now be re-processed to recover the coal contained within it. In addition to recovering the economic value of the coal, re-mining of abandoned gob piles is also beneficial environmentally. As the pile is reprocessed, the waste can be re-disposed in such a way as to minimize water quality impacts such as acid draining or sediment loading into receiving waters.

Figure: Gob piles have historically been environmental problems with coal mines in Appalachia. Today, they are being re-mined and reclaimed, with economic and environmental benefits.