Sämtisersee is a karst lake.

Sämtisersee / A karst lake
The picturesque Sämtisersee is located above Brülisau in the municipality of Rüte in the canton of Appenzell Innerrhoden. The Appenzell mountain lake is located at the foot of the Hoher Kasten in the Alpstein region at an altitude of around 1200 meters above sea level. The Seealpsee and the Fälensee, the other two larger lakes in the Alpstein, are each about three kilometers (as the crow flies) to the southwest. The Sämtisersee is fed by various streams, especially the Sämtiserbach
Text / Source: schweizersee.ch
schweizersee.ch/saemtisersee

The Sämtiersee in the Alpstein Mountains in the canton of Appenzell Innerrhoden is located at an altitude of 1209 meters and usually has a water surface of 13,000 square meters.
As the Alpstein Mountains are made of limestone, the water level of the lake fluctuates in extreme summers when the Sämtiserbach does not have enough water. The water in the lake seeps into the limestone through cracks, crevices, cavities and fissures.

In keeping with the picturesque view of Lake Sämtis, this information board on the geological hiking trail explains its formation