Underground mining is more challenging and requires more miners, but some of the best coal is underground. Underground mining is used when the coal is buried several hundred feet or more below the surface. Some mines can extend to depths of more than 1,000 feet. Miners use heavy machinery to cut out the coal and rely on conveyor systems to transport the coal to the surface. Some underground mines require elevator shafts to move miners and coal to and from the surface. Mining has become much safer and more efficient over the years. Underground mine operations accounted for 69 percent of coal production in Kentucky in 2015, with room and pillar systems being the most common mining method. Throughout most of history, underground mines have provided most employment and coal production in the Commonwealth. During 2015, combined coal production from underground operations and surface operations was more than 61 million tons with a slight majority of production in western Kentucky