In all, at least 140 lignite mines have been flooded to create artificial “pit lakes” across Central Germany. Formerly an opencast mine that closed in 1993, Lake Geiseltal is currently the largest artificial lake in Germany and spans around 18 square kilometers (7 square miles). Lake Geiseltal helps reflect sunlight to the grapevines and the moisture in the air around the lake helps protect them from frost in the winter. An open mine until 1999, Lake Zwenkau is currently the largest pit lake in Neussenland at around 10 square kilometers (4 square miles). Lake Zwenkau was the site of an operational mine until 1999; now it is the largest pit lake in Neussenland at 10 square kilometers (4 square miles).