According to the SEC’s glossary of mining terms, room-and-pillar mining is a method of mining flat-lying ore deposits in which the mined-out area, or rooms, are separated by pillars of approximately the same size.
According to Wikipedia, room-and-pillar is a mining method in which the mined material is extracted across a horizontal plane, creating horizontal arrays of rooms and pillars.
To do this, “rooms” of ore are dug out while “pillars” of untouched material are left to support the roof overburden.
Room and pillar mining can be advantageous because it reduces the risk of surface subsidence compared to other underground mining techniques. It is also advantageous because it can be mechanized, and is relatively simple.
However, because significant portions of ore may have to be left behind, recovery and profits can be low. Room and pillar mining was one of the earliest methods used, although with significantly more man-power.
The room and pillar system is used in mining coal, gypsum, iron, and uranium ores, particularly when found as manto or blanket deposits, stone and aggregates, talc, soda ash, and potash.