Tensions Under the Baltic Sea

The Baltic Sea has emerged as a hot spot for critical undersea infrastructure (CUI) damage. Incidents there have increasingly focused European attention on suspected gray-zone aggression, potentially by Russia and its partners. In the latest case on Christmas Day, an undersea electric cable, EstLink 2, connecting Estonia and Finland, was damaged, along with four telecommunication cables. Finnish authorities are investigating a United Arab Emirates-owned LLC oil tanker, the Eagle S, which is sailing under the Cook Islands’ flag, for having caused the incident. Just over a month earlier, two submarine communication cables, one connecting Germany and Finland and the other Sweden and Lithuania, were severed, allegedly by a Chinese cargo ship, the Yi Peng 3. And in October 2023, the Balticconnector subsea gas pipeline and two telecommunication cables were damaged. Estonian and Finnish investigations focused on the Hong Kong-flagged, Chinese-registered container ship, Newnew Polar Bear, as the culprit.

These events, suspiciously, have much in common. All three cases involve commercial vessels dragging their anchors on the seabed, and all the vessels were sailing from Russian ports. 

The most recent incident adds to growing Western anxiety. The Eagle S is suspected of being part of a Russian “shadow fleet”, a network of ships, many of which in poor seafaring condition, used to evade Western oil sanctions. If confirmed, this would be the first time that a shadow fleet vessel is implicated in deliberately damaging European critical infrastructure. Baltic Sea nations had already raised alerts about threats posed by the fleet, including an increased risk of oil spills.

Quicker Action 

Over the past year, European responses to and investigations into undersea incidents have become faster and more thorough. They have also featured better international coordination. As incidents reoccurred, relevant evidence has accumulated while the statistical probability of repeated accidents has narrowed. European allies have been swifter in labelling the actions “sabotage”. 

After the initial incident involving the Newnew Polar Bear, the ship sailed on uninterrupted, ignoring efforts by local naval forces to contact it. Finland and Estonia opened a criminal investigation into the case and obtained a promise for cooperation from Chinese authorities, but no evidence of a deliberate plot, which would be difficult to prove, has been released.

A year later, a Danish naval patrol was quick to shadow the Yi Peng 3 from the Baltic Sea to the Kattegat Strait, where it was stopped and remained anchored for more than a month. Sweden, the lead investigating country, requested the relocation of the ship to its territorial waters, but this did not occur. The investigation instead proceeded near Danish waters under Chinese oversight. Chinese authorities allowed Finnish, Swedish, German, and Danish counterparts to inspect the Yi Peng 3 but did not permit the Swedish prosecutor to board. The vessel soon resumed its journey. Chinese acceptance of a formal Swedish request for cooperation is pending.

Most recently, once the EstLink2-cable damage was identified, the Finnish Border Guard, within hours, stopped and boarded the Eagle S and then escorted it into Finnish territorial waters. Eight crew members are so far subject to travel bans, and the vessel remains under detention, suspected of aggravated criminal mischief and aggravated interference of communications. In addition, Estonian officials now aim to spearhead maritime legal reforms within the UN’s International Maritime Organization to close loopholes in the Convention on the Law of the Sea and allow for more calibrated responses to CUI disruptions. Finland is also emphasizing the importance of "finding solutions" in maritime law.

Moscow in the Crosshairs

NATO warned in 2023 of a “significant risk” posed by Russia to European and North American critical infrastructure, stating that the Kremlin was “actively mapping” such infrastructure. The United States issued its own warning in September 2024, noting that Moscow was likely to target CUI. European officials have also warned of the Kremlin’s hybrid warfare. Publicly available evidence on Russian involvement in the Baltic Sea cable-cutting incidents, however, remains inconclusive, and European leaders have so far avoided making accusations. Still, Moscow’s broader and increasingly aggressive subthreshold campaign against civil and commercial European assets and stakeholders raises suspicion.

Other nations’ potential participation in these events is even murkier. If the Kremlin proves to be behind the incidents, it seems unlikely that other powers would be involved in such gray-zone operations without some form of earlier tacit agreement or subsequent diplomatic fallout. For nations on which Russia is dependent, such as China, such cooperation would be particularly counterproductive. Nevertheless, NATO must grapple with ways to hold third states accountable, discourage their future involvement, and limit the potential for all parties to claim plausible deniability, challenges made more complex by the role of commercial vessels.

To address gray-zone activities, the EU introduced, only a few days before the latest disruption, a 15th package of sanctions against Moscow and those enabling it to flout Brussels’ previous penalties. The European Commission and the EU high representative for foreign affairs are now planning to propose additional measures on Russia’s shadow fleet. They also aim to boost CUI resilience by enhancing cooperation, undersea repair capabilities, information sharing, and detection technologies. British, Danish, Swedish, Polish, Finnish and Estonian maritime authorities have agreed to crack down on suspected vessels by requesting proof of insurance as they pass through northern European waters. Lastly, the UK-led Joint Expeditionary Force has activated the Nordic Warden system, which aims to boost surveillance of the Russian shadow fleet and prevent CUI disruption. The operation highlights irregular naval maneuvers by providing real-time, continuous, AI-supported surveillance and immediately reports suspicious behavior.

Upping the Ante

Baltic Sea nations and their allies have developed more decisive procedures for dealing with undersea cable-cutting incidents. But will their improved responses deter further incidents, or will the civil and military resources devoted to the effort exacerbate gaps elsewhere?

Russia has a high threshold for pain and acceptable risk. Third-party actors, however, may be less bold. Without their participation, Russia’s ability to claim plausible deniability shrinks, raising the potential risks of its actions. A Helsinki court recently denied a request for the release of the Eagle S and its cargo of 35,000 tons of unleaded Russian petrol, making the incident now a financially costly one for all those involved. But economic implications alone are likely insufficient to deter future gray-zone operations as long as perceived strategic benefits outweigh the cost for Russia and other adversaries.