Drøbak Sound
The Drøbak Sound is a sound at the Oslofjord narrows between Drøbak and Hurum. Outer Oslofjord, which is a term for the Oslofjord south of the Drøbak Sound until it meets the Skagerrak. Inner Oslofjord which is a term for the rest of the fjord, that starts in the Drøbak Sound northwards towards Oslo, where the fjord makes a turn and continues to the Bunne Fjord.
The Drøbak Sound was previously guarded by Oscarsborg Fortress. During the German invasion of Norway on 9 April 1940, the German cruiser was sunk by the fortress. Oscarsborg Fortress has been converted into a museum and a hotel. There is a ferry from Drøbak.
The subsea Oslofjord Tunnel at the Norwegian National Road 23 is running underneath the Oslofjord connects the east side and the west side of the fjord.
