In anthracite coal mines, coal would be mined out in shafts about 10 feet across. The miners would then skip a section of coal and mine out another 10 foot shaft. This method would leave a pillar of coal to provide support for the ground above the mine workings.
A highly dangerous practice in mining was known as "robbing the pillars". This was very profitable for the coal company because it allowed them to take the maximum possible amount of coal from the mine.
When the miners had taken as much coal as they could from the vein, they would mine out the coal pillars which supported the ground above them. The coal pillars would be dug out until either all the coal was gone or the roof caved in, whichever came first. This often resulted in mine subsidence and miners being buried alive due to the lack of support for the ground above.