Precipitation onto land can re-evaporate, infiltrate into the ground, or run off over the ground surface. Water that sinks in the ground may also flow back to the oceans. The oceans are the major sources of evaporation water because of their vast areas of exposed water surface. Most of the fresh water is locked up as ice, mainly in the large polar ice caps. Even the groundwater beneath continental surfaces is not all fresh. Groundwater exists wherever water penetrates through the surface. Groundwater has passed through the rock on aquifer and has been naturally filtered to remove some impurities. Usually, groundwater is found, at most, a few kilometers into the crust. Groundwater is by far the largest reservoir of unfrozen freshwater. This extractable body of groundwater occurs in openings between grains in alluvial and sedimentary rocks – pore water; in fractures usually abundant only near the ground surface – fissure water, and in tubular openings in soluble rocks; and openings in lava formed by flow and gas expansion during solidification – cavern water. The water table is not always below ground surface. Where the water table locally intersects the ground surface, the result may be a lake, a stream, or a spring. Groundwater can flow laterally through permeable soil and rock, from higher elevations to lower, from areas of abundant infiltration to drier ones, or from areas of little groundwater use toward areas of heavy use. The processes of infiltration and migration through which groundwater is replaced are called water recharge. A rock that holds enough of water and transmits it rapidly enough to be useful as a water source is an aquifer. Many of the best aquifers are sandstones or other coarse, clastic sedimentary rocks, but any other type of rock may serve if it is sufficiently porous and permeable – a porous limestone, fractured basalt, etc.