Room-and-pillar mining has been used in the United States longer than any other underground method. Mining is accomplished by driving entries off the panel entries. As mining advances, rooms are excavated in the coal seam, the mountain above the seam is supported by pillars of coal left in place. After a block panel or section has been mined, part of the coal in the pillars can be recovered as a retreat is made toward the main entry. Around 1950, continuous mining using electric-powered machines to bore, dig, or rip the coal from the working face has largely replaced conventional mining. Conventional mining involved undercutting, drilling, placing explosives, and blasting to extract the coal. Coal is either loaded directly into shuttle cars by the machine or in a separate operation. Continuous mining is interrupted by stops to support the roof, waiting of shuttle cars, advance power and water supplies, and service the equipment.