Arnsberg is near the dam of the Möhne Reservoir. During World War II, the dam was breached on 16-17 of May 1943 during Operation Chastise. The Royal Air Force used the infamous dambusters to open the dam. Arnsberg received widespread damage due to the surge of water from the dam and many people were drowned. Abbey Himmelpforten was completely washed away.  Today Arnsberg is a city of about 73 000 people in the Sauerland . In 1975, 12 surrounding municipalities were merged into Arnsberg. The city itself was first mentioned in documents dating back to 789.