A spring is a naturally occurring flow of water from the ground. This may be, for example, an outflow from an underground stream; but most often a spring occurs where an aquifer saturated groundwater intersects with the Earth's surface. Such an aquifer, if confined above and below by aquicludes, may travel for hundreds of kilometers underground before emerging at the surface, there, perhaps, in desert areas giving rise to oases. Spring water is generally fairly clean, since it has been filtered through the permeable rocks; but all spring water contains some dissolved minerals.