geopiet Mar 9, 2015:

Underground Mining

Two dominant methods together account for the vast majority of underground coal mining: “room and pillar” and “longwall” mining.
In room and pillar mining, seams of coal are mined partially, leaving large pillars of coal intact to support the overlying layers of rock. Mining by this method creates a network of alternating open spaces and large pillars of coal. When mining in one part of the coal seam has been finished, miners practice “retreat” mining, extracting as much as possible of the coal in the remaining pillars on the way out, while the roof of the seam begins to collapse behind them. - http://goo.gl/Z0gLKe