The Drøbak Sound (Norwegian: Drøbaksundet) is a sound at the Oslofjord narrows between Drøbak and Hurum. Outer Oslofjord, a term for the fjord south of the Drøbak Sound until it meets the Skagerrak, starts there. The Drøbak Sound was previously guarded by Oscarsborg Fortress. During the German invasion of Norway on 9 April 1940, the German cruiser Blücher was sunk by the fortress. Oscarsborg Fortress has been converted into a museum and a hotel. There is a ferry from Drøbak.