Groundwater is the source of more than 80 % of the water supply of Switzerland. For this reason, an urgent task of both the public and administration is to seek a sustainable exploitation and optimal protection of groundwater resources. An aquifer is a geological formation in which a large amount of groundwater circulates through openings. In Switzerland, it is possible to distinguish roughly between six major types of aquifers. Representative examples are schematically illustrated by means of cross-sections and diagrams on the map page.