is a liquid circulated through the borehole during rotary drilling; it is used to bring cuttings to the surface, cool and lubricate the drill stem, protect against blowouts by holding back subsurface pressures, and prevent fluid loss by plastering the borehole wall. Mud formulations originally were suspensions of clay or other earth solid in water, but today are more complex mixtures of liquids (not necessarily water), reactive solids, and inert solids.

Also see drilling oil.