Groundwater is not in underground lakes, nor is it water flowing in underground rivers. It is simply water that fills pores or cracks in subsurface rocks. When rain falls or snow melts on the surface of the ground, some water may run off into lower land areas or lakes and streams. What is left may be absorbed by the soil, seep into deeper layers of soil and rock, or evaporate into the atmosphere.
Below the topsoil—the rich upper layer of soil in which plants have most of their roots—is an area called the unsaturated zone. In times of adequate rainfall the small spaces between rocks and grains of soil in the unsaturated zone contain at least some water, whereas the larger spaces contain mostly air. After a major rain, however, all the open spaces may fill with water temporarily. During a drought, the area may become drained and almost completely dry, although a certain amount of water is held in the soil and rocks by molecular attraction.