The Eichener See ("Lake Eichen"), known in the Alemannic dialect as the Eiemer See, near Eichen in the Baden-Württemberg county of Lörrach is a periodic (astatic) karst lake in the Southern Black Forest in Germany.
The lake, which only contains water when there is a high water table, lies in a hollow, a muschelkalk-karst basin. It has no surface inflow, its outflows are partly subterranean and partly through evaporation.
Karst lakes are formed as the result of a collapse of caves, especially in water-soluble rocks such as limestone, gypsum and dolomite. This process is known as karstification. They can cover areas of several hundred square kilometres. Their shallow lakebed is usually an insoluble layer of sediment so that water is impounded, leading to the formation of lakes. Many karst lakes only exist periodically, but return regularly after heavy rainfall.