Frank has some good points but also to point out that most subsidences happen in workings that are close to the surface, this is caused by, running water, freezing and thawing ground, small sizemic events, rain water seeping through to name a few. we have seen a few places where subsidences into workings were only 8-10 feet below the surface. in steeply pitching coal mines subsidences usually happen where the coal face deteriorates and it is close to the surface causing a crop fall, or chute opening to the surface. many mines actually did this for ventilation purposes. in flat pitch mines they are usually caused by simply mining too close to the surface. in archbald there was a subsidence directly below the rr tracks, and there is a manway entrace near. so we went in the manway then saw where they filled the subsidence with concrete. that was pretty cool. check this out....
http://www.undergroundminers.com/subsidenceindex.html