In October 1981, the Soviet submarine
S-363 accidentally hit an underwater rock about 10 kilometres (6.2 mi) from the main Swedish naval base at Karlskrona, surfacing within Swedish waters. The boat's presence coincided with a Swedish naval exercise, testing new equipment, in the area.
Swedish naval forces reacted to the breach of sovereignty by sending an unarmed naval officer aboard the boat to meet the captain and demand an explanation. The captain initially claimed that simultaneous failures of navigational equipment had caused the boat to get lost (despite the fact that the boat had already somehow navigated through a treacherous series of rocks, straits, and islands to get so close to the naval base). The Soviet Navy would later issue a conflicting statement claiming that the boat had been forced into Swedish waters due to severe distress, although the boat had never sent a
distress signal, and instead attempted to escape
As the Soviet captain was being interrogated, the weather turned bad and the Soviet submarine sent a distress call. In Swedish radar control centres, the storm interfered with the radar image. Soviet jamming could also have been a factor. As the Soviet submarine sent its distress call, two ships coming from the direction of the nearby Soviet armada were detected passing the 12-nautical-mile (22 km) limit headed for Karlskrona.
This produced the most dangerous period of the crisis and is the time where the Swedish Prime Minister Thorbjörn Fälldin gave his order to "Hold the border" to the Supreme Commander of the Swedish Armed Forces, General Lennart Ljung. The coastal batteries, now fully manned as well as the mobile coastal artillery guns and mine stations, went to "Action Stations". The Swedish Air Force scrambled strike aircraft armed with modern anti-ship missiles and reconnaissance aircraft knowing that the weather did not allow rescue helicopters to fly in the event of an engagement. After a tense 20 minutes, General Ljung called Prime Minister Fälldin again and informed him that it was not Soviet surface ships but two German merchant ships.
The boat was stuck on the rock for nearly 10 days. On 5 November it was hauled off the rocks by Swedish tugs and escorted to international waters where it was handed over to the Soviet fleet.