Permeable rocks absorb the rainwater, and little by little they get filled up. Once they are completely filled, we say that they are saturated. The saturated permeable rocks form the phreatic layer . The subterranean waters are stored in aquifers, as the layer is commonly called. For this, underneath the porous rocks they have a lower layer of impermeable rocks that impede the passage of water, stopping it and helping it to be stored.
Fresh water is found in rivers, lakes, ponds, in subterranean waters, and in the atmosphere.
Nevertheless, in conformity with the size of our world, and in spite of how necessary it is for life, in reality we have very little fresh water. Large natural deposits are found in the glaciers of Greenland and the Antarctic and in Lakes of North America or Russia. Humid, tropical zones contain high percentages of the total world reserve. In other zones, it is necessary to take advantage of every drop in order to survive. It has been calculated that a man needs 50 liters or more daily in order to satisfy his personal and home needs.