Ground water, water accumulated beneath the earth's surface in the pores of rocks, spaces, cracks, etc. Most underground water originates as precipitation that sinks into soil and rocks. Permeable, water-bearing rocks are aquifers; rocks with pores small enough to inhibit the flow of water through them are aquicludes. Buildup of groundwater pressure beneath an aquiclude makes possible construction of an artesian well. The uppermost level of groundwater saturation is the water table.