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 20m. Fish can survive only in the top 18m, as the water below 20m is salty, although bacteria and worms have been found below 20m that can live without oxygen.
During 1943-44, 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. They also fired torpedoes from a launching pad in the lake into the Tote Mountains, producing vast holes in the canyon walls. Millions of counterfeit pound sterling notes (£100+ million) were dumped in the lake after Operation Bernhard, which was never fully put into action. There is speculation that there might be other valuables to be recovered from the bottom of the Toplitzsee. The speculation is due to there being 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 reports that he saw a sunken aircraft below this layer.