Submarines, some of unusual design, attacked the Swedish coastal defence system in the Baltic several times in the 1980s, deliberately damaging electronic sensors on the sea bed, Swedish authorities have confirmed. The Swedes have not solved the mystery of the prowling submarines, as they were unable to confirm who was responsible.
A special Submarine Commission acknowledged some reports of "alien submarine activity" detected by sonar were, in fact, swimming minks.
From 1981 to 1994 there were more than 6,000 reports of submarines intruding into Swedish coastal waters, peaking in 1987 and declining thereafter to "a couple of hundred" in 1994. Sweden has a complex system of coastal defences, using coastal artillery, mines and missiles linked to underwater sensors. In 1981 Sweden protested after a Soviet Whiskey class submarine, U-137, ran aground in south-eastern Sweden near a top-secret naval base.
The Russians blamed a navigation error, but the Commission confirmed that the submarine entered Swedish waters deliberately and had been carrying nuclear weapons. However, it said the evidence for the other penetrations of Swedish defences did not justify blaming them on the Russians.
In the mid-1980s a mined area off northern Sweden was tampered with and put out of action. The Commission said electric equipment at great depth had been damaged by blows from a hard object. The nature of the damage and the depth indicated it was planned and carried out with sophisticated equipment.
Although some reports were explained by mink, others were clearly caused by man-made submarine activity. In October 1982, passive sonar detected a submarine in the Danziger inlet, and in 1988 and 1992, active sonar detected "alien underwater craft" in the Havringebukten area. On both occasions the craft were estimated to be be about 100ft long - smaller than a conventional submarine. In May 1988 a submarine was detected in the Gothenburg archipelago.
Among the mysteries were tracks on the sea bed, suggesting the country responsible, assumed to be Russia, was using unorthodox submarine vehicles, which could also drive along the sea-bed, either to inflict specific damage on Swedish defences or as part of some test programme.
Tracks were found on the seabed at Harsfjarden in 1982, at Karpelshamnsviken in 1987 and Havringebukten in 1988.Reuse content