Water returning to the earth's surface as precipita- tion takes various paths. Most precipitation falling on terrestrial ecosystems becomes surface runof. This water flows into streams and lakes, which eventually carry water back to the oceans, from which it can evaporate to repeat the cycle. Some surface water also seeps into the upper layer of soils and some evaporates from soil, lakes, and streams back into the atmosphere. Some precipitation is converted to ice that is stored in glaciers, usually for long periods of time. Some pre- cipitation sinks through soil and permeable rock forma- tions to underground layers of rock, sand, and gravel called aquifers, where it is stored as groundwater.