Kreidesee Hemmoor

The chalk lake in the Warstade district is the legacy of 117 years of cement production. Chalk was mined here from 1862 to 1976 and processed in a cement factory directly on what is now the lake. After the end of mining, the pit filled with groundwater. The lake, originally 130 meters deep, was flattened to 60 meters by backfilling rubble and rubble from the original chalk factory, but is still the third deepest body of water in northern Germany and covers around 33 hectares.The lake is also internationally known as a diving and leisure center and is a challenging diving area due to the depth, visibility and low water temperature. There have already been fatal diving accidents several times. The lake can be dived by divers all year round for a fee; a dive center is on site. Carrying a second first stage with a separate regulator is mandatory. In addition to the sunken industrial plant, underwater attractions include car wrecks, a sunken sailing boat and a sailing yacht, a truck that was brought in on the "shaker" and an airplane, a Piper PA-28, which formerly belonged to the astronaut Alan Shepard.Various types of trout and salmon are bred in the mostly crystal-clear water, which can also be fished.
Source: de.wikipedia.org/wiki/Hemmoor#Kultur_und_Seehensw%C3%BCrdigkeit