The "Nonnenmattweiher" is a small corrie lake. Its bed was formed by a glacier during the last ice age that ended approximately 10,000 years ago. In Medieval times the lake silted up and became a moor. In 1758 a dam was built to hold back the water and run a mill that was located further down the valley. Then something very special happened. The flooded moor floated up and formed a swimming peat island. On March 1, 1920, after prolonged heavy rains, the dam broke and the flood devastated most of the upper part of the adjoining "Kleines Wiesental" valley. In June, 1934, a new dam was officially opened (or closed rather) and the moor once again floated up and an even larger swimming peat island formed itself.

The lake is divided into two parts by a barrier of swimming logs. In the panorama you can see the part to the left which is open for swimmers while the part to the right is under strict nature conservancy. The flat area to the very right is the swimming peat island.