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.
"Nonne" is also a word for a nun and there is an old lore explaining the name of this place accordingly: It is said that there was an old monastery on that very location which, after God's judgement, was drowned in the lake together with all its inhabiting nuns.