A general term that refers to a mass mining system where the extraction of the ore depends largely on the action of gravity. By removing a thin horizontal layer at the mining level of the ore column, using standard mining methods, the vertical support of the ore column above is removed and the ore then caves by gravity. As broken ore is removed from the mining level of the ore column, the ore above continues to break and cave by gravity. The term "block caving" probably originated in the porphyry copper mines, where the area to be mined was divided into rectangular blocks that were mined in a checkerboard sequence with all the ore in a block being removed before an adjacent block was mined. This sequence of mining is no longer widely used. Today most mines use a panel system, mining the panels sequentially or by establishing a large production area and gradually moving it forward as the first area caved becomes exhausted. The term "block caving" is used for all types of gravity caving methods. There are three major systems of block caving, and they are differentiated by the type of production equipment used. (1) The first system based on the original block cave system is the grizzly or gravity system and is a full gravity system wherein the ore from the drawpoints flows directly to the transfer raises after sizing at the grizzly and then is gravity loaded into ore cars. (2) The second system is the slusher system, which uses slusher scrapers for the main production unit. (3) The last system is the rubber-tired system, which uses load-haul-dump (LHD) units for the main production unit. Block caving has the lowest cost of all mine exploitation systems, with the exception of open pit mining or in situ recovery.