This source, located in Pasvalys, on the left bank of the River Lėvuo, is the Lithuania's deepest cave, and its currently known depth is 20 m. The source from the cave flows into the River Lėvuo. The spring gushed from a sinkhole in 1960. The ground water erupting from the bottom of the sinkhole snapped in a floodplain several meters long bed, which the bottom is covered with light grey, almost greenish sediments, issuing the spring water greenish tint. In its sinkholes, we can see some pieces of gypsum and dolomite. The source water is clear and cold with an odour of hydrogen sulphide. It is the largest source of Pasvalys district and the deepest cave in Lithuania. The Green Spring is included in the Lithuanian Book of Records “Factum”. In 2006, the underwater research began. In 1985, the Green Spring was declared as a natural monument.