Lake Toplitz (German: Toplitzsee) is a lake situated in a dense mountain forest high up in the Austrian Alps, 98 km (61 mi) from Salzburg in western Austria. It is surrounded by cliffs and forests in the Salzkammergut lake district, within the Totes Gebirge (dead mountains). The Toplitzsee water contains no oxygen below a depth of 20 m. Fish can survive only in the top 18 m, as the water below 20 m is salty, although bacteria and worms that can live without oxygen have been found below 20 m.

In 1943 and 1944, the shore of Lake Toplitz served as a Nazi naval testing station. Using copper diaphragms, scientists experimented with different explosives, detonating up to 4,000 kg charges at various depths. Over £100 million of counterfeit pound sterling notes were claimed to have been dumped in the lake after Operation Bernhard, which was never fully put into action. In 1959, investigators recovered £700 million of counterfeit notes from the lake, that Hitler had planned on using to sabotage Britain's economy. There is speculation that there might be other valuables to be recovered from the bottom of the Toplitzsee. There is a layer of sunken logs floating half way to the bottom of the lake, making diving beyond it hazardous or impossible. Gerhard Zauner, one of the divers on the 1959 expedition, reported that he saw a sunken aircraft below this layer.

The area is only accessible on foot, via a private mile-long track that serves the Fischerhütte (Fisherman's Hut) restaurant at the western end.