Recent repeated acts of sabotage of undersea infrastructure call for a coordinated and collective response. The EU must forge a global coalition to strengthen protections for underwater critical infrastructure, pushing for a more robust interpretation of maritime law to safeguard cables and pipelines. Member states must deploy innovative solutions, involving enforcement of national regulations on maritime and environmental safety and improved information sharing with partners around the world, while NATO should expand its deployment of unmanned systems and sensors.
Rising threats to the undersea infrastructure
Over the past year, there have been a troubling series of incidents of apparently intentional damage to undersea infrastructure in the Baltic Sea. In October 2023, the Balticconnector gas pipeline, which links Finland and Estonia, was damaged by the Chinese vessel ‘Newnew Polar Bear’. In November of the following year, ‘Peng 3’, a Chinese bulk carrier, severed two communication cables: the BCS East-West Interlink connecting Lithuania and Sweden, and the C-Lion 1 cable between Finland and Germany. The most startling event occurred on Christmas Day 2024, when the ‘Eagle S’, an oil tanker registered in the Cook Islands, cut a power cable Estlink 2.
While initial responses from authorities in the affected coastal states were relatively muted, Finland took a far more assertive approach in the case of the Eagle S. Concerned by the growing wave of underwater sabotage and its potential to destabilise regional security and connectivity, the Finnish Coast Guard ordered the vessel into its territorial waters, where it was boarded by law enforcement. The ship was subsequently seized, and criminal investigations were launched into potential sabotage and related crimes.
The Estlink 2 incident was a turning point in the discussions on the security of critical undersea infrastructure in the Baltic Sea. Recognising that a collective and coordinated response is needed, NATO Secretary General Mark Rutte co-hosted a Summit of Baltic Sea Allies in January 2025, along with Finnish President Alexander Stubb, and Estonian Prime Minister Kristen Michal. This led to the launch of the Baltic Sentry mission under the authority of NATO’s Supreme Allied Commander, designed to deter attempts to damage critical undersea infrastructure in the region. The mission aims to bolster situational awareness through enhanced maritime patrols, utilising frigates, patrol aircraft and naval drones to monitor the seas and ensure the security of vital infrastructure.
Can Baltic Sentry deter and punish?
After the summit, Stubb said the actions taken by Finnish authorities with the Eagle S underscored NATO’s commitment to "act robustly" whenever critical infrastructure in the Baltic Sea is threatened. Baltic Sentry is aimed at enhancing the Alliance's ability to employ both deterrence by denial (preventing incidents from occurring) and deterrence by punishment (imposing consequences on perpetrators) to protect undersea cables, pipelines, and other vital underwater infrastructure.
While the mission’s objectives are clear and sound, there are two significant challenges to full implementation: a lack of maritime capabilities, both manned and unmanned, and limited legal authority of states to act in international waters, particularly when it comes to stopping and boarding foreign flagged vessels.
Cutting undersea cables is a classic example of a hybrid operation. In most cases, it does not meet the criteria for an armed attack. This makes it a "below-threshold" activity, where states struggle to respond robustly. Pinpointing the exact source of such actions, often carried out by non-state actors, poses a major attribution challenge both on the legal and diplomatic levels, especially when suspicions point to Russia. Ambiguity around the origin and intent of these attacks provides ample room for plausible deniability for any suspected state actor. Moreover, the operation is highly asymmetrical, as an attacker needs little more than an old vessel and an anchor, while the defender must commit significant resources to safeguard the infrastructure.
Too many ships, too few eyes – plugging the technical gaps of maritime surveillance
The Baltic Sea is a critical artery for global trade with 15% of the world’s container shipping passing through its waters – more than 4,000 vessels per day. Monitoring such a dense maritime corridor and ensuring that none of these ships poses a threat to the vast network of underwater infrastructure is a major task. Traditional naval surface assets, such as frigates, are not sufficient for such an enormous undertaking, especially as European NATO navies have shrunk in recent decades, leaving the Alliance without enough vessels to patrol these waters (beyond national boundaries) effectively.
To bridge this gap, NATO is increasingly turning to unmanned systems and underwater sensors. This shift aligns with its recently unveiled Digital Ocean Vision, an initiative designed to enhance maritime surveillance and situational awareness "from seabed to space." By integrating national and Allied capabilities, including underwater robots, aerial drones and space-based assets, NATO aims to bolster its ability to monitor and secure maritime infrastructure. According to NATO’s Allied Command Transformation (ACT), 20 unmanned surface vehicles (USVs) have already been committed to the Baltic Sentry operations.
However, the challenges remain with the number and capabilities of current unmanned underwater vehicles (UUVs). The operational depth of these vessels is limited due to environmental pressure and low communication range, preventing them from reaching the seabed, where most cables and pipelines are located. Addressing this requires deeper cooperation and coordination between NATO and the EU. In this context, advanced maritime monitoring and surveillance capabilities developed under the European Maritime Safety Agency, such as CleanSeaNet and the Copernicus Maritime Surveillance service, can play a key role.
The EU has already included the development of “great depth enabling technologies” and an “advanced underwater network” to interconnect multiple UUVs, sensors, and other civilian and military underwater assets as main priorities of the 2025 workplan of the European Defence Fund.
Given their inherent suitability for joint ownership and command, maritime assets should be prime candidates for collective development and deployment within NATO. A proven model already exists in NATO’s Airborne Warning and Control System (AWACS), which operates under a shared multinational structure. A similar approach could be applied to unmanned maritime capabilities and sensors, strengthening joint surveillance, deterrence, and the Alliance’s ability to safeguard critical underwater infrastructure.
Sea lawfare – weaponisation of the law of the sea
One of the key challenges facing the Baltic Sentry mission, as well as individual state efforts to deter attacks on undersea infrastructure, is the legal constraints imposed by the law of the sea. Under customary international law and the United Nations Convention on the Law of the Sea (UNCLOS), states cannot interdict or board ships on the high seas, including within their exclusive economic zones (EEZs), even if they have compelling evidence that a vessel has deliberately severed an undersea cable linked to their country. Both Sweden and Finland asked the suspected ships to sail to their respective territorial waters and only there proceeded to board. It is unclear how Stockholm and Helsinki would have acted had the shipmasters ignored these demands.
The only recourse under UNCLOS is to seek compensation at a court in the vessel’s flag state. However, this is a complicated process due to the opaque ownership structures of certain vessels. This approach offers no real deterrent effect, leaving states with limited options to either prevent attacks on critical underwater infrastructure or seek compensation for such actions.
States orchestrating such actions are effectively engaging in lawfare, abusing legal rules to restrict countermeasures. In this case, they weaponise one of the fundamental principles of the law of the sea: the right to freedom of navigation. By exploiting this legal shield, an attacker can act with near impunity, knowing that even a vessel suspected of cutting an undersea cable cannot be interdicted on the high seas without violating international law. This gives hostile actors an asymmetric advantage in hybrid operations at sea.
The UNCLOS treaty also guarantees the right of states to lay and operate undersea cables – not just in their territorial waters, but also on the high seas. Any deliberate attack on these cables can therefore be considered a violation of this right and a wrongful act, allowing the victim state to take proportional countermeasures under the law of state responsibility.
Strengthening legal protection of undersea infrastructure
One potential countermeasure could be the temporary seizure of a suspect vessel until accountability is established and compensation secured. While such an interpretation of UNCLOS has never been formally tested, the EU and its member states should push for international recognition of this approach, rallying global support for stronger legal protections of undersea infrastructure. Pipelines and cables are critical lifelines, particularly for small island states, and their protection under international law must be more than just theoretical.
The EU should also consider strengthening the enforcement of international and national maritime regulations within the territorial waters of Baltic coastal states. Many of the ships involved in cutting undersea cables, such as Russia's shadow fleet circumventing sanctions, are often old and poorly maintained. To address this, suspected vessels should be monitored using sniffer drones, capable of detecting hazardous leaks like oil or petrol.
Denmark, with its strategic location in international straits, is well positioned to lead such operations. Although freedom of navigation is guaranteed in these sea lanes, they are also Danish territorial waters. Therefore, Denmark has the authority to enforce environmental and safety regulations. The EU should also work with coastal states in other corners of the world, notably Malaysia, Indonesia and India, to exchange information about these vessels, helping to mitigate the risk of potential environmental disasters like oil spills and share best practices on law enforcement actions.
Legal and practical avenues exist to tackle underwater sabotage in the Baltic – provided NATO states are willing to invest in advanced technologies, particularly drones and sensors, and adopt a more assertive interpretation of the law of the sea to safeguard the freedom to lay and operate undersea cables.
The EU and NATO should also seek broader international backing for this legal stance while anticipating and preparing strategies to counter potential risks of escalation. This includes measures to shield Denmark and other affected states from possible Russian or Chinese retaliation, as well as addressing scenarios where Western ships might be detained on trumped-up pretexts in a sort of maritime hostage-taking.
This approach would not only enhance the resilience of critical undersea infrastructure but also reinforce the rules-based order against growing hybrid threats.
Juraj Majcin is a Policy Analyst at the European Policy Centre
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