Swedish sightings of Russian submarines

Swedish sightings of Russian submarines have become a kind of seismograph for security tensions in northern Europe. From the dramatic grounding of a Soviet boat outside Karlskrona in 1981 to today’s reports that Sweden encounters Russian submarines in the Baltic almost weekly, these episodes illuminate both the evolution of Russian undersea power and Sweden’s changing strategic position, culminating in her recent entry into NATO. 

We consider the history of such sightings, the submarine types most plausibly involved, and what Moscow is trying to achieve when her boats slip in or close to Swedish waters.

Geography, neutrality and the undersea problem

Sweden’s coastline runs for some 2,700 kilometres, fringed by a labyrinth of skerries and islands. The waters around the Stockholm archipelago and the approaches to Karlskrona are shallow, broken, and full of acoustic clutter. This environment is well suited to small, quiet diesel-electric submarines and deeply challenging for sonar. It was precisely this geography that made Sweden both an attractive target for clandestine Soviet operations during the Cold War and a difficult place in which to prove what had really happened once a suspicious echo appeared on a sonar screen. 

During the Cold War Sweden was formally neutral, yet she was closely aligned with western powers in practice. From Moscow’s perspective that created a strong incentive to probe Swedish defences, map minefields and approaches to naval bases, and prepare for any future conflict in which NATO might exploit Swedish territory or waters. The Baltic Fleet’s submarines were natural tools for this work.

The Cold War: from “Whiskey on the Rocks” to the submarine hunts

The S-363 / U-137 incident

The defining moment in Swedish-Soviet undersea confrontations came on 27 October 1981, when a Soviet Project 613 “Whiskey” class diesel-electric submarine, S-363, ran hard aground on rocks near Karlskrona, well inside a restricted Swedish naval area. Swedish sources designated her U-137. 

The boat had somehow navigated through narrow channels and a minefield laid to protect the naval base. Later Swedish analysis suggested that she was likely engaged in covert reconnaissance of this defensive belt rather than merely lost. To make matters more serious, Swedish measurements of radiation indicated that S-363 carried nuclear-armed torpedoes. 

For ten days Sweden and the Soviet Union faced one another in a tense standoff. Swedish forces surrounded the stranded submarine, while a Soviet task group moved towards the area to support her. Prime Minister Thorbjörn Fälldin famously ordered the Swedish military to “hold the border” as Stockholm insisted upon inspecting the vessel before allowing her to leave. 

From the point of view of submarine types, S-363 represented an older coastal patrol design rooted in Second World War German technology. Whiskey-class submarines were relatively small and noisy by modern standards, but they were well suited to Baltic operations and had the range and endurance to operate along the Swedish coast. 

The broader pattern of 1970s–1980s incidents

The Karlskrona drama was merely the most visible episode in a broader pattern of suspected submarine incursions. Swedish archives record a surge of “submarine incidents” from the mid-1970s into the late 1980s, involving sonar contacts, periscope sightings and occasional photographs of small masts or hulls in archipelagic waters. Some Swedish defence reports spoke of forty or more probable intrusions per year at the height of the tension. 

Amongst these, the 1982 Hårsfjärden incident, in which Swedish forces dropped depth charges and mines against an unidentified submarine in a naval exercise area, became particularly controversial. Despite intense searches, no wreck was ever recovered. Subsequent debate has ranged from those who consider it almost certainly a Soviet intruder to scholars who argue that at least some contacts in this era were misidentified natural or civilian phenomena, or possibly even NATO submarines. 

The Soviet types most likely to have been involved, where intrusions really occurred, were again diesel-electric coastal designs. Alongside Whiskeys, analysts have suggested that early Kilo-class submarines, Tango-class boats and small special-forces delivery submarines could have been used. These were capable of operating in shallow water and of inserting reconnaissance groups close to Swedish naval facilities. The evidence for specific hulls is fragmentary, yet the pattern of Soviet interest in Sweden’s defences, revealed so starkly by S-363, makes such operations entirely plausible.

After the Cold War: fewer boats, lingering shadows

With the end of the Soviet Union, both Swedish and Russian navies contracted sharply. Funding for Swedish anti-submarine warfare declined. Russian submarine patrol rates fell as maintenance problems proliferated. Yet reports of suspicious contacts did not cease altogether. The historical record suggests a combination of genuine Russian activity, misinterpretation of sensor data, and the inertia of institutional memory in Sweden, where officers had spent much of their careers hunting Soviet submarines. 

During the 1990s and early 2000s the Baltic Fleet retained several Kilo-class boats, purpose-built for anti-ship and anti-submarine warfare in shallow coastal waters. Kilos are relatively quiet when running on battery power and difficult to track, especially in complex littorals such as the approaches to Stockholm. 

Russian resources, however, were limited. Many hulls spent long periods tied up at pier, whilst crews struggled to maintain proficiency. This relative lull in Russian naval activity lulled some Swedish opinion into believing that the submarine threat had faded. Events after 2014 would reverse that impression.

The 2014 “foreign underwater activity” and its aftermath

In October 2014 the Swedish armed forces launched a large operation in the Stockholm archipelago after receiving intelligence about “foreign underwater activity”. Corvettes, minesweepers and helicopters, supported by some 200 personnel, spent nearly a week searching for what was widely believed to be a small foreign submarine or submersible. 

The Swedish military later stated that there had been a confirmed territorial violation by an underwater craft, based upon sonar recordings and other classified data, although no vessel was brought to the surface. Russian officials denied involvement and at one point sought to deflect attention by suggesting that a Dutch submarine had been misidentified, which The Hague firmly rejected. 

Open sources point to several possibilities. One is that a small Russian special-purpose submarine or swimmer delivery vehicle was operating close inshore, potentially testing Swedish response times or inserting reconnaissance teams. Another is that an ordinary diesel-electric submarine from the Baltic Fleet was operating unusually far inside the archipelago. Either way, the incident reawakened Swedish memories of the 1980s and underlined that Russian undersea activity in the Baltic had not disappeared.

Since Russia’s annexation of Crimea in 2014, sightings and encounters with Russian naval units in the Baltic, including submarines, have become more frequent. Swedish officials now describe contacts with Russian submarines as an almost weekly occurrence. At the same time there have been suspicious incidents affecting undersea cables and pipelines in the wider Baltic region, which western officials frequently connect to Russian intelligence or sabotage operations, whether carried out by submarines, special-purpose surface vessels or unmanned systems. 

Russian submarine types operating in or near Swedish waters

Kilo and Improved Kilo

The workhorse of Russia’s non-nuclear submarine force is the Kilo family. The original Project 877 Kilo class is a diesel-electric boat optimised for coastal operations, with a low acoustic signature and the ability to lie quietly on the seabed whilst listening for targets. 

Modernised variants, particularly the Project 636.3 “Improved Kilo”, have been fitted with better sonars and the capacity to fire Kalibr cruise missiles, which can strike ships or land targets hundreds of kilometres away. These submarines have been heavily used in the Black Sea and Mediterranean, but some Kilos belong to the Baltic Fleet and periodically transit through the Skagerrak and Kattegat, passing relatively close to Sweden’s west coast.

In wartime these boats would be tasked with interdicting NATO shipping, threatening ports such as Gothenburg, and, crucially, menacing undersea infrastructure. Their presence in or near Swedish waters in peacetime provides crews with essential training in the same geography in which they would be expected to fight.

Lada and other new diesel-electric designs

Russia has also sought to introduce the Project 677 Lada class, a more advanced diesel-electric submarine intended to replace older Kilos. Although the Lada programme has faced serious delays, the small number of hulls completed are also designed for operations in constricted, shallow seas such as the Baltic. Over time similar boats, sometimes referred to under the prospective Kalina label, may form a modernised coastal squadron that would naturally operate in the vicinity of Swedish waters.

Nuclear attack submarines and the wider approaches

Strictly speaking, large Russian nuclear-powered attack submarines such as the Yasen class are unlikely to spend time inside the Baltic itself, given the sea’s limited depth and the constraints of the Danish straits. Nevertheless Yasen boats in the Norwegian Sea or North Atlantic can operate “off” Swedish waters in the broader sense, observing and potentially threatening the flow of allied reinforcements that would be critical to Sweden and her neighbours in a crisis. 

These nuclear boats are armed with long-range Kalibr and other cruise missiles which can reach targets in Scandinavia from far offshore. Their role complements that of the smaller diesel-electric craft closer to Sweden’s coasts.

Special-purpose submarines and mini-subs

Finally, Russia maintains a portfolio of special-purpose submarines and small submersibles designed to deploy special forces, tamper with seabed infrastructure, or conduct covert reconnaissance in harbours and archipelagos. The 2014 Stockholm incident, in which some evidence pointed towards a small semi-submersible craft, illustrates how such platforms might be used. 

These platforms are particularly relevant to Sweden because they are well suited to narrow inshore waters and can operate where even a compact Kilo would risk running aground, as S-363 once did.

Purposes of Russian submarine missions near Sweden

Intelligence collection and mapping of defences

The most enduring purpose of Russian submarine activity around Sweden is intelligence. The S-363 incident almost certainly involved an attempt to map the protective minefields and approach routes to Karlskrona. In the same vein, modern Kilo-class or smaller special-purpose submarines can collect acoustic signatures of Swedish and allied warships, identify the locations of passive sonar arrays, and chart safe channels through shallow waters.

Such mapping is not merely academic. In wartime these data would allow Russian submarines to reach positions from which they could threaten Swedish ports, NATO reinforcement routes or maritime approaches to the Baltic States. The more detailed the peacetime reconnaissance, the lower the risk in crisis.

Testing readiness and exerting psychological pressure

Repeated intrusions or close approaches to territorial waters also serve to test Swedish readiness and reaction patterns. When Sweden surges helicopters and corvettes in response to a suspected submarine, Russian planners can observe which units deploy, how quickly they arrive, and how their sensors are used. The 2014 hunt, which required significant Swedish resources but yielded no captured submarine, provided Moscow with a wealth of information. 

At the same time, such incidents impose psychological pressure. In the 1980s, Swedish public opinion was shocked by the revelation that a Soviet submarine with nuclear torpedoes had been inside a key naval base. In the 2010s, the inability to present a captured boat after dramatic searches exposed Swedish politicians and officers to domestic criticism, whilst reinforcing the sense of vulnerability. This kind of pressure can be used to influence debates about defence spending, nuclear policy or, in the longer term, alliance choices.

Ironically the cumulative impact of these episodes, in combination with Russia’s broader aggression against Ukraine, helped to push Sweden towards abandoning neutrality in favour of NATO membership, which is hardly the outcome Moscow would have preferred. 

Hybrid warfare and threats to seabed infrastructure

Modern Russian submarine missions also fit into a wider pattern of hybrid warfare. Undersea cables carry the overwhelming majority of digital communications, whilst gas pipelines and power interconnectors traverse the Baltic Sea. Western governments increasingly worry that Russia could use submarines, special-purpose submersibles, or unmanned underwater vehicles to monitor, interfere with, or sabotage this infrastructure in crisis or war. 

Russian Kilos equipped with modern cruise missiles are additionally capable, at least in principle, of targeting infrastructure nodes such as offshore platforms or coastal power stations. Routine patrols near Swedish waters therefore contribute to the practical and doctrinal preparation for such options, even if individual missions are not overtly aggressive.

Strategic signalling and alliance politics

Submarine deployments can also be read as signals. By keeping her Baltic Fleet submarines active near Sweden Russia demonstrates that, despite setbacks elsewhere, she retains the reach to threaten NATO’s northern flank. Publicised transits of Kilo-class boats through narrow straits, often monitored by Swedish and Danish ships, are a way of showing the flag in reverse, below the surface rather than above it.

Such signalling is directed not only at Sweden but at NATO more broadly. It reinforces the message that the Baltic Sea remains contested and that any future conflict would rapidly involve the underwater domain.

Training and routine patrols

Finally, not every sighting reflects a special mission. Submarine crews need regular time at sea to maintain skills in navigation, silent running and weapons employment. The Baltic Sea provides a convenient training ground for diesel-electric submarines stationed in Kaliningrad or Saint Petersburg. Some portion of the “almost weekly” encounters reported by Swedish officers are likely to be these routine patrols at the edge of, or occasionally inside, Sweden’s maritime zones. 

From Sweden’s point of view, however, there is no such thing as a harmless foreign submarine inside her territorial waters. Each contact must be treated as a potential threat, which in turn drives Stockholm to invest in anti-submarine capabilities and closer cooperation with allies.

Consequences for Sweden and for NATO

The cumulative effect of decades of submarine scares has been profound. The 1980s incidents led Sweden to strengthen her coastal surveillance, invest in modern corvettes and helicopters with advanced sonar, and refine territorial defence doctrines. 

The 2014 operation and the broader deterioration of relations with Russia contributed to a new wave of investment, including seabed sensors and participation in multinational anti-submarine exercises. After joining NATO Sweden has taken part in alliance drills such as joint Baltic anti-submarine exercises, intended partly to signal that the days when Russian submarines could loiter unnoticed in her archipelagos are over. 

For NATO Swedish waters and airspace are now central to any plan for the defence of the Baltic States or the Arctic. Russian submarine activity near Sweden is no longer a bilateral nuisance but a challenge to the alliance’s collective ability to protect critical infrastructure and maintain sea lines of communication.

Swedish sightings of Russian submarines are more than mysterious periscopes in the mist. From the grounded Whiskey-class S-363, carrying nuclear torpedoes within sight of Karlskrona, to the elusive contacts that sparked the 2014 submarine hunt, and the present pattern of near-weekly encounters with Kilo-class patrols, they chart the changing balance of power in the Baltic and the evolution of Russian undersea doctrine. 

The submarine types involved have shifted from crude coastal patrol boats to modern cruise-missile carriers and specialist mini-subs, but their purposes remain broadly continuous: mapping defences, gathering intelligence, testing readiness, exerting psychological pressure, and preparing options for coercion or attack against infrastructure and shipping. Sweden’s response has moved in the opposite direction, from solitary neutrality to embedded alliance membership, precisely because the country has grown convinced that she cannot manage this undersea challenge alone.

For so long as Russia fields submarines in the Baltic and adjacent seas, and for so long as Sweden sits astride key maritime chokepoints and seabed cables, their shadow contest under northern waters will continue. What changes after Sweden’s NATO accession is not the presence of periscopes off the coast, but the network of allies now watching for them.