The underground waters are part of the hydrologic water cycle that includes the continuous water movement between the earth and the atmosphere by means of the evaporation and the precipitation. Part of the water that falls by the precipitation of rain and snow goes to lakes, rivers, streams and the oceans. Another part is absorbed by the vegetation, which transpires the water again towards the atmosphere. The water that does not evaporate directly of the lakes and rivers or is transpired by the plants, filters across the subsoil and becomes part of the aquatic undergrounds.