The glacier lake on the slope of Plechý mountain (below a 220 m high rock wall) is situated at the altitude of 1090 m, and has a surface of 7,48 hectares, circumference of 1242 m and the deepest point at 18,30 m. The lake retains water in the volume of 617 000 cubic metres.
Similarly to the other Šumava glacier lakes, Plešné also originates from the epoch of Pleistocene. Over a kilometre long glacier on the slope of Plechý hollowed out a corrie and piled up a 30-40 m high moraine. This depression was filled with water of melting glacier in the Interglacial.
The lake has no inflow, only an effluent in form of the Jezerní stream, which empties into the Vltava river. The face moraine was tightened in 1789-1791 in respect with construction of the Schwarzenberg Floating Canal. At the same time, 2,5 m high dam with gates was built into the moraine to enable regulated outflow from the lake to feed the Schwarzenberg Canal with water. Up to 177 thousand cubic metres of water in total could be drained of the lake.
Plešné Lake was a popular tourist destination in the early 20th century already, which is why a tourist cottage called Lucemburská was built here that later became a property of the Club of Czech Tourists. The building was nevertheless seized by the Frontier Guard and adapted for its purposes after 1948. Later on the Frontier Guard abandoned the building and the former tourist cottage was demolished in 1989.