It is called the Eye of the Eichsfeld (the region it is located in) – the lake Seeburger See named after the city Seeburg next to it. You can use this natural lake for boating and swimming but it is rather flat with a maximum depth of 3.5 meters. Better test upfront how deep it is before you jump into the water. 😉 The Seeburger See is a famous tourist destination and you will find good infrastructure at its shore.
Best known is the restaurant Graf Isang but there is also a camping area, a kiosk, boat renting companies and different places for swimming. Be reminded that the area is a natural protection zone since 1973. If you’re at Göttingen for example you can also reach the lake by bike – a nice one hour ride if you don’t mind to take the steep and long Roringer Berg. But there are also enough car parking areas available.