Groundwater is an underground treasure Groundwater is formed from precipitation that seeps into the soil. The water is only stopped by water-impermeable rock layers such as clay or loam. There it accumulates and becomes groundwater. Almost a third of the freshwater resources are stored underground in this way. Groundwater is an important source of drinking water! On its way through the different layers of rock, the water is filtered and purified. It's also picking something up: Minerals that are released from the rock layers. Groundwater is very clear and clean water!