As the word implies, groundwater is water held in saturated zones below the land surface. For many, this brings to mind rapidly coursing waters running through cavernous areas underground. In a relatively few cases, that image is correct. But for the most part, a sponge would make a better analogy than an underground river.
Evaporation returns water to the atmosphere to complete the never-ending cycle.
Diagram from Sponenberg, Torsten D. and Jacob H. Kahn, A Groundwater Primer for Virginians , Virginia Water Resources Research Center, 1984. Though our language makes a clear distinction between surface water and groundwater, the line between the two is not always a bright one. In the words of writer Joe Gelt, The classifications seem clear enough, surface water occurring above ground, ground-water found below ground. A belief in surface water here and ground-water there simplifies the making of laws and policies. Nature, however, was not designed for easy, simple regulation.