When water falls on the ground as rain, hail or snow, it can soak into the ground - this process is called infiltration. Water that soaks in this way may be stored underground as groundwater. Some percolates down to the water table. Other water may become trapped in sand or porous rock such as sandstone or limestone. Rock formations containing useable quantities of groundwater are called aquifers.
How much water soaks into the ground depends on: the heaviness of the rainfall the land's slope the soil type the soil conditions (is it dry or already wet?) the amount and type of vegetation cover.