Water covers more than seventy percent of the Earth’s surface but less than three percent is potable sweet water . Sweet water is essential for life but in many cases it is also a risk factor endangering life and property. Sustainable use of this limited, vulnerable resource is one of the most important and difficult tasks of our century. As a result of water circulation, water is in continuous transformation and movement. We have known for some decades that water movements are organised in well-definable systems in non-visible underground regions, too. We have understood that such water movements taking place on a geological time-scale systematically transform both the underground and overground environment, including habitats. To facilitate the sustainable use of water resources, we need to understand the functioning of hydrological systems. In addition, information about underground water flows can be used in many areas, from agriculture, through environment protection to the mapping of thermal waters.