well, aperture in the earth's surface through which substances in a natural underground reservoir, such as water, gas, oil, salt, and sulfur, can flow or be pumped to the surface. In the United States, until some years after the Civil War, the majority of wells were "open," i.e., holes dug in the ground and lined, or cased, with brick, stone, or wood. Although they are sometimes dug with picks and shovels, most wells today are made by rotary or percussion drills. An artesian well, the most desirable type of water well, is always drilled because rock layers must be cut through to reach the water. Oil wells are usually drilled using a rotary-drill method, in which a drilling bit set in the bottom of a drilling pipe is rotated by machinery on the ground level. As the cut deepens, more sections of pipe are fastened to the sections already in use. A special mixture called drilling mud is sent down through the pipe to wash away the drillings and also to cool the cutting bit. Some oil wells are drilled by a percussion method known as cable-tool drilling. In this procedure a heavy metal bit attached to a cable is alternately raised and dropped, pulverizing the rock beneath it. Water is pumped into the well and mixed with the rock cuttings, the mixture being bailed out when it becomes thick enough to interfere with the action of the bit. Regardless of the drilling method, well walls are usually cased with iron or steel to prevent cave-ins. Casing is inserted when the desired depth has been reached or, in some instances, as the well is being drilled. Minerals, such as salt and sulfur, can be pumped to the surface through a well if they are first liquefied by some process; for example, salt may be brought up if water is first pumped to the bottom of the well to dissolve the salt. In natural gas wells drilled in relatively narrow bands of shale, horizontal drilling is used in combination with hydraulic fracturing, or "fracking," in which pressurized fluids are injected into the well to induce rock fractures that permit the release of greater quantities of gas.