Groundwater is stored in the water table

Everybody knows that the ground can soak up water. Even though it seems solid, the top layer of the earth's surface contains many tiny holes which can store water, just like a sponge. The lowest layer, with all of its spaces filled with water, is called the "zone of saturation." Above this layer is the "zone of aeration," in which some of the holes are filled with air instead of water. The boundary between these two layers is the "water table." In order to get groundwater from a well, the well must be drilled below the water table, into the zone of saturation. The depth of the water table varies greatly from place to place, depending on both the composition of the ground and the amount of rainfall the area receives. Groundwater makes up less than 2% of the Earth's total water, but it is still the primary source of water for many people around the world.


Credit: Hailey King, NASA GSFC

For more info: http://earthobservatory.nasa.gov/Library/Water/water.html