torpedo battery, at Kvarven Fort, in that it was designed to launch its torpedoes from under the water level, instead of by torpedo tube from above ground as was the case with the battery at Kvarven outside Bergen. At Oscarsborg the torpedo battery is a concrete construction inside a cave mined into the rock of the North Kaholmen island. Two torpedoes are loaded side by side, in two open steel frames. Then one of the two frames is lowered like an elevator down into the water to the tunnels below. After one shot, it took some time to swap frames and be ready for the next. When fired, the torpedo's own compressed air engine was started and it propelled itself. The battery has three torpedo tunnels which could fire six torpedoes without reloading and a total of nine torpedoes was stored and ready for use. Each weapon carried a 100 kg TNT warhead and targets were spotted from three observation bunkers just above the battery. The torpedoes were delivered in 1900 from the Whitehead torpedo factory in Fiume, then part of Austria-Hungary. A back-up observation bunker is situated just outside the entrance to the battery. Second World War When Norway was invaded on 9 April 1940, all of the fortress' armament was over 40 years old, and of German origin. Both the guns and the torpedo battery worked flawlessly when Oscarsborg encountered one of the German invasion flotillas; they sank the heavy cruiser Blücher, and threw back the German naval force heading for Oslo, thus managing to save the Norwegian King and government from being taken prisoner. During the occupation of Norway, German forces were stationed at Oscarsborg. The fortress was returned to Norwegian control on 12 May 1945 when Captain Thorleif Unneberg took command of the fortifications and raised the Norwegian flag following the capitulation of all German forces in Norway four days earlier. The flag in question was the same that had flown over the fortress until it was captured by the Germans in April 1940. Colonel Birger Eriksen was present on the occasion and made a speech. After the Second World War During the Cold War Oscarsborg formed a last line of defence for Oslo, with the underground torpedo battery remaining secretly active up until 1 January 1993, having been modernized in the 1980s. After the deactivation of the last weapons systems, the remaining military activity on