Altausseer See, Alpine lake near Loser Mountain, Austria.

Altausseer See is an alpine lake in Austria surrounded by steep mountain walls and known for its clear waters. The shoreline features wooded areas and open access points, while the deeper water takes on a deep blue color that reflects the surrounding peaks.

The lake formed roughly 16,000 years ago through glacial movements that carved deep basins in the Totes Gebirge mountains. This massive geological transformation shaped the landscape visible to visitors today.

Every third year the Narzissenfest fills the Seewiese with figures and sculptures made from the white Narcissus radiiflorus, which blooms in the meadows around the hunting lodge in late May. During the festival a rowing-boat parade crosses the lake, and the Seewiese hunting lodge, now a restaurant, holds a small natural history museum on the lake and its fossils.

A 7.4 km loop path, the Uferpromenade, circles the lake; in winter the stretch between the Strandcafé and Kaltenbrunn is closed because of avalanche and rockfall risk. Public bathing jetties stand near the Seeklause car park and the lakeside park in Altaussee. The lake is reached by the Altausseerstraße L702, and combustion engines on boats are banned.

After heavy rain, the Liagern phenomenon can raise the lake level by up to 60 cm within 12 hours. Water surges from up to four karst springs about 30 m above the valley floor near the Seewiese, forming roaring waterfalls that reach the lake after a very short run.