Many people think that groundwater is filled with soil, plant roots, and bugs; however, the very opposite is true. Groundwater is one of the cleanest and purest sources of freshwater on Earth. Freshwater becomes groundwater when water infiltrates the Earth and moves downward through pore spaces and cracks in the soil and rock. Once the water encounters an impermeable surface, it will collect in the pore of the rock, thus creating an aquifer. The groundwater system contains two zones. The zone of saturation is an area where water collects. The zone of aeration is the permeable area just above the zone of saturation. Located between the two zones is the water table. Since groundwater is such a valuable source of freshwater, humans rely on the resource and acquire it through digging wells and pumping the water up to the surface. Humans must be careful not to over-use the groundwater. Over-use can drop the water table and even cause wells to run dry.