The water table is not flat, however. It more or less conforms to surface topography but in a somewhat subdued fashion. This is important because it means that groundwater can flow from higher to lower areas. If all the pore spaces that are filled with water in the subsurface of the U.S. were connected, they would form one large water-filled cavern, 57m high, underlying the entire country. The higher the porosity, generally, the more stored groundwater.
Primary porosity refers to that which developed when the rock formed.
Secondary porosity develops after the rock is formed. Primary porosity is usually higher in sedimentary rocks and lower in igneous rocks. It can be high in volcanic rocks, however. Porosity generally decreases with depth. The deepest drillhole on earth, the Kola Hole in Siberia, at a depth of 12km did have water-filled fissures.