The Swedes are currently searching for a mysterious underwater activity in the Stockholm archipelago. It comes on the tail of a joint exercise which may be the motive for the intrusion. Some reports suggest it may have been an underwater surveillance system (hydrophones etc) and others that it may have been an eye witness. If it is the latter then hopefully they had the presence of mind to whip out their camera-phone. A Dutch submarine participating in the exercises has been ruled out.
News reporting of the incident. Source: SVT News
Background
Throughout the 1980s the Swedes responded to a large number of incursions into their territory. The most reported and clear-cut case was the 1981 'Whiskey on the Rocks' incident caused when a Soviet WHISKEY class diesel submarine beached itself off Sweden's main naval base at Karlskrona. Many incidents followed, and although some may have been false alarms or civilians, the finger was pointed squarely at the Soviets.
In the days before phone cameras there was very little hard evidence which left a hole for alternative explanations to emerge. There have been many theories and not everyone accepts that it was the Soviets. At the end of the Cold War the incidents more or less stopped, but in recent years the Russian Military has been emboldened in the Baltic. In recent times the Russian Airforce has led the charge, until now.
Suspects
As the news channels follow the latest developments on the current submarine search, I am wondering which types of submersibles and Submarines are actually present in the Baltic. This s very much a first pass analysis based on limited news available. Assuming that it is not a false alarm:
I think that the most likely candidates are indeed the Russian Navy. The Baltic fleet has some modern surface combatants but as a submarine fleet is secondary to the Northern and Pacific fleets and is generally equipped with less capable equipment. But it still has three diesel submarines and Spetsnaz. The special purpose submarines like Project 1851 X-RAY, Project 18511 PLATUS, Project 1910 UNIFORM and Project 210 LOSHARIK are all based outside the Baltic and are not likely candidates.
The newest submarine in the Baltic is the single Project 677 Lada Class boat Sankt Peterburg (B-585). This is a modern SSK. Also present are two Project 877 KILO Class boats which are older but still considered formidable. In my view any of these three are equally likely candidates either as the interloper or as the mother-boat.
Lada Class and KILO Class SSKs
The well-known LOSOS Class midget submarines have long since been discarded and no equivalent submarine is currently thought to be in service so it is unlikely to be a midget submarine. The Spetsnaz still have some SDVs and diver propulsion devices which could be used to extend the range of frogmen. These include TRITN-1M (I doubt if any TRITON-2 are still operational) and submersible boats which have sufficient range to operate independently (not the TRITON-NN which does not appear to be in service).
Contemporary Russian SDVs: Sirena SDV, Triton-1M SDV (may be retired), Proton SOM-1 DPV and modular Proton SDV.
The TRITON-NN submersible boat is not believed to have entered service
Conspiracy fodder
If you were to suppose that it might be a NATO country, either in league with the Swedes to cause an incident for political gain, or unintentionally, then the German and Polish Navies are most obviously equipped. Britain and US only have SDVs which could fit the bill and to be honest I don't think this is at all likely.
NOTE: I DO NOT THINK IT IS NATO
German U212 Class submarine, prototype Gabler two-man submarine launched SDV and in-service Kampfswimmer 2-man SDV
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