Wikipedia
Plötzensee is a small glacial lake in Berlin. It is situated near the Rehberge public park in the former borough of Wedding, now a part of Mitte. The name stems from Plötze, one name for the roach in German, as the lake formerly teemed with it. Plötzensee is part of a chain of lakes stretching from the northeast to the Spree valley, formed in the last ice age. Until 1443 the nunnery of St. Mary at Spandau had the rights to the lake, but these were eventually assumed by the Prussian treasury. In 1817, the city of Berlin bought the lake and leased the rights to the shoreline and fishing.
The first public bath opened about 1850 and over the years, there have been an army sporting ground, a man-made beach (photo), an inn, and the conversion of the shore into a public park in the 1920s.
The name Plötzensee also refers to Plötzensee Prison, built nearby in the 19th century, which reached its height of notoriety in the time of Nazi Germany.