Lake Lungern (Lungerersee, Lungernsee or Lungerensee) takes its name from the town of Lungern at its south eastern end and is today used as a reservoir. It is 2km long and reaches 800 metres at its widest point. The restaurant at its northern edge offers wonderful views towards the Bernese Oberland which can be seen in the film. Following the road south towards Interlaken, one comes to the Brünig Pass and from there one descends down to Brienz.
The lake is drained by the Sarner Aa river, which flows through the Sarnersee and into Lake Lucerne.
The lake was originally much larger, and covered a large part of the valley. In 1836, its level was lowered by 36 metres through an artificial drainage tunnel of 380 m length. One can see in places where the water once reached.