The USGS Water Science School
The ground beneath our feet is not just rock, or at least, not just one kind of rock. Many different types of rock exist, and they have very different properties. Often, different types of rocks exist in horizontal layers beneath the land surface. Some layers are more porous than others, and at a certain depth below ground the pores and fractures in these rocks can be totally filled with water (an aquifer). When precipitation falls and seeps into the ground, it moves downward until it hits a rock layer, which is so dense and unfractured that it won't allow water to easily move through it. When this happens it is easier for the water to start moving horizontally across the more porous rock layer. Sometimes when building a road, the layers are cut into and revealed, and water can be seen dripping out through the exposed layers.
Ground water Aquifers Ground-water seepage