mixture of clay, usually bentonite, and water used primarily in oil, gas, and water-drilling operations to carry rock cuttings to the surface and also to lubricate and cool the drilling bit. Special chemicals are added to the mud to compensate for the varying composition of the water and the formation being drilled and to increase the weight of the column. The drilling mud, by hydrostatic pressure, also helps prevent the collapse of unstable strata into the hole and the intrusion of water from water-bearing strata that may be encountered.