Grundloser See

The bottomless lake isn't that bottomless ...
It was probably one of the last secrets of Schleswig-Holstein. How deep is the bottomless lake? The popular destination at Hessenstein in the municipality of Panker has never been officially measured. Henning Thiessen from the Lütjenburg Ice Age Museum made up for this. The result is sobering.
At the deepest point in the middle, the water measures only 3.50 meters. In the bank area, the plumb bob did not sink deeper than 1.50 meters. The Grundlos lake is not nearly as deep as its name might suggest. The Grundlos Lake is a dead ice hole from the last ice age, which ended 12,000 years ago. At that time there was a huge iceberg at the place of the lake, which reached deep into the earth. When the subterranean structure melted, a kind of crater developed that filled with water. Thiessen assumes that the Grundlose See could actually have been 20 or 30 meters deep when it was young. From the beginning, it is surrounded by forests whose leaves blew into the water. The soil layer grew over the millennia.
Source and further information:kn-online.de/Lokales/Ploen/Panker-Grundloser-See-doch-nicht-so-grundlos

The lake is not without reason. It was once a so-called dead ice hole and is said to have been an estimated 20 meters deep.