This web page provides an overview of USGS's research about groundwater. It includes related research, publications, software, and a collection of data and tools. Groundwater is water in the ground that fully saturates pores or cracks in soils and rocks. Water underlies the Earth's surface beneath oceans, hills, valleys, mountains, lakes, and deserts. It is not always easy to get to or clean enough for use without treatment, but it exists essentially everywhere if you dig deep enough. Groundwater may occur close to the surface or it may lie many hundreds of feet below. And not all groundwater is the same age... water at very shallow depths could be just a few hours old, whereas water at great depth may have been there for several thousand years (Introduction).