Another item from the November issue (issue 280) of the Fortean Times magazine, pages 18 and 19. This may, or may not, relate to the UFO phenomenon, depending on your point of view.
The article starts off by noting that "There were more than 4,000 reported detections of foreign submarines in Swedish waters in 1982-1992..." The article goes on to note that "There was a great USO (unidentified submarine objects) 'flap' in 1983 and Swedish ships regularly dropped depth charges into Baltic waters..."
"Some regard the USOs as extraterrestrial; perhaps travellers from a watery planet were exploring the seabed for intelligent life..."
Mostly though it was human submarines, from the former USSR though "A number of the intrusions into Swedish waters in the 1980s were allegedly carried out by the US and the UK, partly to train submarine crews, partly to keep Swedish defence forces on their toes, and partly as a psychological operation."
Sightings continue right up to 2011, e.g. (click here.)
For an indepth look at USOs in Norwegian and Swedish waters click here.