Oslofjord Tunnel

The Drøbak Sound (Norwegian: Drøbaksundet) 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 Blücher 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 runs underneath the Sound and connects the east side and the west side of the fjord.

Oscarsborg Fortress (Norwegian: Oscarsborg festning) is a coastal fortress in the Oslofjord, close to the small town of Drøbak in Viken County, Norway. The best known part is situated on two small islets. The main artillery batteries are on the island Håøya and smaller batteries on the mainland to the west and east in the Fjord and was military territory until 2003 when it was made a publicly available resort island. The fortress is best known for sinking the German heavy cruiser Blücher on 9 April 1940. In 2014, Oscarsborg Fortress was given protected status.

Drøbak is a town and the centre of the municipality of Frogn, in Viken county, Norway. The city is located along the Oslofjord, and has 13,409 inhabitants.

Skiphelle is a village in Frogn, Akershus, Norway.

Blücher was the second of five Admiral Hipper-class heavy cruisers of Nazi Germany's Kriegsmarine (lit. 'War Navy'), built after the rise of the Nazi Party and the repudiation of the Treaty of Versailles. Named for Gebhard Leberecht von Blücher, the Prussian victor of the Battle of Waterloo, the ship was laid down in August 1936 and launched in June 1937. She was completed in September 1939, shortly after the outbreak of World War II. After completing a series of sea trials and training exercises, the ship was pronounced ready for service with the fleet on 5 April 1940. She was armed with a main battery of eight 20.3 cm (8.0 in) guns and, although nominally under the 10,000-long-ton (10,000 t) limit set by the Anglo-German Naval Agreement, actually displaced over 16,000 long tons (16,000 t). Immediately upon entering service, Blücher was assigned to the task force that supported the invasion of Norway in April 1940. Blücher served as the flagship of Konteradmiral (Rear Admiral) Oskar Kummetz, the commander of Group 5. The ship led the flotilla of warships into the Oslofjord on the night of 8 April, to seize Oslo, the capital of Norway. Two old 28 cm (11 in) coastal guns in the Oscarsborg Fortress engaged the ship at very close range, scoring two hits, as did several smaller guns in other batteries. Two torpedoes fired by a torpedo battery in the fortress struck the ship, causing serious damage. A major fire broke out aboard Blücher, which could not be contained. The fire spread to one of her anti-aircraft gun magazines, causing a large explosion, and then spread further to the ship's fuel bunkers. Blücher then capsized and sank with major loss of life. The wreck lies at the bottom of Oslofjord, and in 2016 was designated as a war memorial to protect it from looters. Several artifacts have been raised from the wreck, including one of her Arado 196 floatplanes, which was recovered during an operation to pump out leaking fuel oil from the ship in 1994.

Heer is a village in Frogn, Akershus, Norway.

The Battle of Drøbak Sound took place in Drøbak Sound, the northernmost part of the outer Oslofjord in southern Norway, on 9 April 1940. It marked the end of the "Phoney War" and the beginning of World War II in Western Europe. A German fleet led by the cruiser Blücher was dispatched up the Oslofjord to begin the German invasion of Norway, with the objective of seizing the Norwegian capital of Oslo and capturing King Haakon VII and his government. The fleet was engaged in the fjord by Oscarsborg Fortress, an aging coastal installation near Drøbak, that had been relegated to training coastal artillery servicemen, leading the Germans to disregard its defensive value. However, unbeknownst to German military intelligence, the fortress' most powerful weapon was a torpedo battery, which would be used to great effect against the German invaders.The fortress' armaments worked flawlessly despite their age, sinking the Blücher in the sound and forcing the German fleet to fall back. The loss of the German flagship, which carried most of the troops and Gestapo agents intended to occupy Oslo, delayed the German occupation long enough for King Haakon VII and his government to escape from the capital.

Frogn is a municipality in Akershus county, Norway. It is part of the Follo traditional region. The administrative centre of the municipality is the town of Drøbak. Frogn was established as a municipality on 1 January 1838 (see formannskapsdistrikt). The city of Drøbak was merged with Frogn on 1 January 1962. Frogn is located at the southern part of the peninsula between the main Oslofjord and Bunnefjorden. It borders Nesodden, Ås and Vestby.

Håøya is an island in Oslofjorden in Frogn municipality, Norway. Its area is 5.6 km², which makes it the largest island in Inner Oslo Fjord. It is located between Oscarsborg and Gråøya. Søndre Håøya is a nature reserve area. White-tailed eagles nest on the southeastern part of the island. They returned in 2008 after an absence of 126 years. Visitors are permitted to harvest mushrooms and berries under 'freedom to roam' but other plants and flowers are protected. Goats graze the island during the summer. The island has a mooring jetty, camping areas, swimming beaches, public toilets, cabins for rent, and historic bunkers and buildings. The island was part of the defense of Oslo, along with Oscarsborg Fortress.

The Hurum air disaster was an Aero Holland plane crash in Hurum southwest of Oslo, Norway when a Douglas DC-3 which was carrying Jewish children from Tunisia who were to transit through Norway while immigrating to Israel crashed as it was approaching Fornebu Airport on 20 November 1949, killing 34 people, including 27 children.