The cirques at the high altitude zone contain 189 glacial lakes; there are also about 30 smaller ones that evaporate in summer, including tectonic lakes like Panichishte. Their location is closely linked to the snowline during the last glacial period and most of them lie at altitudes between 2,100 m and 2,500 m. Most lakes (28) are situated at altitudes between 2,300 m and 2,350 m; there are 23 between 2,350 m and 2,400 m, 19 between 2,250 m and 2,300 m and 19 between 2,400 m and 2,450 m. Their length varies between 800 m and 20 m, the width — between 375 m and 10 m, the area — between 1 m and 212 decares, the depth — between 0.5 m and 37.5 m. Most of them are between 2 m and 10 m deep and four reach depth of over 20 m. The water is transparent as deep as 15 m. Most of the lakes are covered with ice during most of the year — from October to June. The ice thickness reaches 3 m in the highest lakes. The largest glacial lake in Rila and in the Balkans is Smradlivo with an area of 212 decares; the longest is the Upper Ribno Lake reaching 801 m; the deepest is Okoto, one of the Seven Rila Lakes with a depth of 37.5 m; the highest is Ledeno at an altitude of 2,709 m; the lowest is Suho at 2,045 m. The most important lake groups are the Seven Rila Lakes (seven lakes), Musala Lakes (seven), Marichini Lakes (seven), Urdini Lakes (six), Malyovishki Lakes (three), Elenski Lakes (three), Chanakgyolski Lakes (two), Vapski Lakes (two), etc.