Groundwater is water that sits under the ground, in the soil and in the small cracks of rock. Rain falls, sinks in, and some of it stays down there, filling the spaces like a hidden sponge. People dig wells to pull this water up for drinking and farming. It is not a river you can see, and it is not the puddle on the road after rain. Though actually, it does move, only very slowly, from one place to another under our feet. If people take too much, or if dirty water soaks down from farms and towns, the groundwater in a whole area can run low or become unsafe — and you would not know from looking at the surface.