Securing the Depths: Rethinking EU Critical Infrastructure Protection in a Contested Underwater Domain

CSDS POLICY BRIEF • 13/2025

By Giacomo Leccese and Ivan Zaccagnini

6.5.2025

Key issues

• Controlling and monitoring the undersea domain is becoming crucial for the European Union, with geopolitical and economic interests at risk in specific sea regions ranging from the Baltic to the Mediterranean.
• The EU’s cable security plan puts the feasibility and security of new European underwater infrastructure at risk due to the legal, political and military implications, especially given Russia’s new assertiveness.
• Instead of fixed cable sensors, Europe should prioritise investing in two alternative and combined solutions: EU-funded projects for unmanned vehicles and innovative fibre optic sensing solutions.

Introduction

“A New Era of Undersea Conflict Is Here”. With this warning, a recent article in Foreign Policy highlights the increasing dangers looming over underwater telecommunications and energy infrastructure. The European Union (EU) seems to be one of the main battlefields of this “conflict”, as demonstrated by the numerous and suspicious damage to submarine cables and pipelines in the Baltic Sea and beyond. The relevance of sabotage of maritime infrastructure for the EU has already been discussed in a previous CSDS Policy Brief, which highlighted how such hostile actions require a rethink of the European way of perceiving international threats. This CSDS Policy Brief focuses on the best responses that the Union can adopt to ensure the protection and security of its critical underwater infrastructures. Specifically, it analyses the implications of adding sensors on the cables, as proposed in the “EU Action Plan on Cable Security”, presented on 21 February 2025 by the European Commission and the High Representative.

Based on four objectives – prevention, detection, recovery and deterrence –, the Action Plan places great importance on using new technologies to monitor the seabed. In particular, the Action Plan calls for the increased adoption of “Smart Cables” in order to boost early-warning by anticipating threats and protecting the infrastructure and the surroundings. Science Monitoring And Reliable Telecommunications (SMART) technology consists of adding temperature, pressure and acceleration sensors on the repeaters placed along a cable to enable sustained monitoring of the seabed. Although designed for environmental purposes, in the Union’s plans this option can also respond to military needs by helping to anticipate and prevent possible attacks on infrastructure. However, as this CSDS Policy Brief will show, the addition of sensors to critical seabed infrastructure gives rise to several political and legal implications. We analyse these challenges and the possible technological alternatives available.

Collecting precious data amidst sovereignty claims

The recommendation to increase the use of SMART cables is part of a broader European prevention strategy focused on resilience, particularly on enhancing network redundancy (i.e. the number of both communication and power connections). Adding sensors to infrastructure, however, risks hindering European projects and creates barriers to implementing new routes. The adoption of SMART cables, despite being advocated by the scientific community since 2010, and having been the subject of a Joint Task Force (JTF) by the UN International Telecommunication Union (ITU), has so far remained particularly challenging. Projects have been limited to the territorial waters of a single state or short routes between two countries. The world’s first SMART cable was completed only in 2024, off the coast of Sicily, Italy, to monitor seismic activity in the Ionian Sea; two more cables are under construction between Vanuatu and New Caledoniaand between the Portuguese mainland and the Azores and Madeira. Aside from a few possible exceptions, like the EU-funded MEDUSA cable, for which the addition of sensors is under consideration, other projects – especially larger ones – have faced challenges due to concerns among some governments about perceived infringements on sovereignty, national security or improper data gathering regarding marine resources.

Sovereignty claims find fertile ground in the challenging legal classification of SMART cables, which do not fall under the category of standard telecommunication cables. Some governments argue that the addition of sensors would place such infrastructure under the scope of Marine Scientific Research (MSR), which is regulated differently under the international law of the sea. Unlike standard cables, which can be laid and maintained freely beyond the territorial waters of the coastal state, MSR requires that states can request permits even within the Exclusive Economic Zone (EEZ) and the Continental Shelf. Therefore, classifying SMART cables as part of MSR may lead governments to deny permission for installation due to security concerns about the data that could be collected. A potential fear involves the possibility that this data might be used to identify and subsequently reprimand states that are not doing their part to curb marine pollution or prevent overfishing. Such a scenario is particularly likely in the Mediterranean, where several non-EU countries are significantly affected by marine pollution and have a substantial environmental impact on the region’s ecosystem. These states may fear and consequently hinder the addition of sensors on cables transiting through their EEZs.

In addition to this logic, the main constraint remains linked to the potential for data collection that could be used for military purposes. Although the JTF on Smart Subsea Cables has voluntarily avoided including the addition of hydrophones that could be used for submarine detection, even the data collected by pressure, temperature and acceleration sensors can have military applications. The detection of sound propagation in water has been and continues to be the primary method for detecting submerged vehicles, but non-acoustic systems can also be used. A significant example is the Systema Obnaruzhenya Kilvaternogo Sleda (SOKS), developed in the late 1960s by the Soviet Navy, which allowed the Soviet Union to trail NATO vessels undetected on several occasions. Through such “wake detection systems” it is possible to detect a submarine through the changes in temperature and pressure in the water column caused by its movement. While these detection methods were particularly difficult in the past, they are now set to play an increasing role in anti-submarine warfare (ASW) due to improvements in processing power and oceanographic modelling, which allow for a quicker and more effective analysis of the collected data. In this context, the collection of scientific data, such as that gathered by SMART sensors, especially if shared publicly as recommended in the case of MSR, could be impeded by states interested in keeping their underwater assets concealed.

From this perspective, there are two specific contexts in which legal disputes could hinder European redundancy plans. The first is, once again, the Mediterranean, where the intersection of growing investments in submarine fleets by several navies and the rivalries between states – often involving countries outside the Union – could discourage the collection and sharing of information that would be useful for detecting underwater military assets. The importance of this “hinder-finder dynamic” was confirmedin the military stand-off between Greece and Turkey in 2020, when the ability to keep their submarines undetected provided a significant military advantage under the sea to the Greeks. The other critical geographical context is the Arctic, which is increasingly important for digital connectivity. Building cables across Arctic waters offers several advantages including reduced latency, being the shortest route between Northern Europe and Asia, and in terms of safety and diversification, allowing passage through regions with little congestion from commercial traffic. European cable projects like Far North Fiber and Polar Connect, which are considering the possibility of smart sensors, could be obstructed by Russia due to Moscow’s territorial claims and military operations in the region. In fact, those sensors could be utilised by Western navies to detect Russian underwater assets, and, more broadly, to understand the operational environment for future missions.

Putting a target on your back?

What has just been described concerns the possible impediments that states could place on the introduction of SMART technologies in the waters under their sovereignty. However, some hostile countries could hinder such projects even outside the limits of the EEZ and Continental Shelf, covertly sabotaging the infrastructure. In this perspective, the addition of SMART technology, rather than safeguarding the infrastructure, could put it at even greater risk. For Europe, the main threat in this sense comes from Russia. Moscow would see some of its interests threatened by the adoption of SMART sensors in some European marine areas, such as the Baltic Sea, the Mediterranean and the so-called “GIUK Gap” (Greenland, Iceland and United Kingdom (UK)).

Since the mid-2010s, the Baltic Sea has increasingly become a critical theatre for Russian naval operations. With its rearmament, the Kaliningrad military base is now a key re-supply and defence hub for Russian submarines and Anti-Access/Area Denial (A2/AD) capabilities. Recently, the centrality and strategic importance of this exclave have become even more evident due to the closure of Tartus naval base in Syria, which occurred in the aftermath of the unrest and the deposition of former President Bashar al-Assad in December 2024. Consequently, the introduction of SMART cables in Baltic waters could be perceived by Moscow as a direct challenge to its maritime mobility and strategic initiatives. Since 2015, Russia has bolstered its Baltic Fleet with advanced assets, including the deployment of Kilo-class diesel-electric attack submarines (SSKs) and the development of a new and improved version – the “Project 636.3” Kilo-class. Since 2023, these submarines have conducted deep dives and training activities in the region. The effectiveness of these platforms, known for their stealth capabilities and versatility, could be compromised by the presence of SMART sensors positioned on the region’s seabed. The new “Project 636.3” Kilo-class SSKs, which can benefit from improved capabilities due to a special anechoic coating and a main propulsion plant isolated on a rubber base to prevent noises and vibrations, could be detected by SMART technology. Furthermore, Russia’s sea denial strategies in the Baltic Sea involve the potential use of naval mines to control access and disrupt maritime supply lines to Baltic states. The presence of SMART sensors could impede such tactics by detecting and deterring clandestine mine-laying activities, thereby undermining Russia’s ability to execute area denial operations effectively.

In the Mediterranean, Russia has demonstrated a strategic interest in maintaining a naval presence, exemplified by its operations in Syria and the Eastern Mediterranean and by the alleged return of its submarines in 2025. It is very likely that Moscow will soon return to be fully operative in the region, be it by regaining the rights to use the Tartus base, negotiating with the new Syrian government or be it in other areas such as Tobruk or Benghazi in Libya, currently under the control of Moscow-backed General Haftar and where Russia is reportedly already moving equipment from Syria. The deployment of SMART cables in this region could interfere with Russia’s naval and intelligence operations. In fact, the Russian Navy has previously utilised the Tartus naval facility in Syria to support these activities, and the installation of advanced monitoring systems like SMART sensors could be viewed by Russia as a hindrance to its logistical and operational flexibility in the Mediterranean.

Furthermore, the Arctic region, particularly the “GIUK Gap”, has regained strategic importance amid increasing Russian naval activity and Arctic expansion. The deployment of SMART cables in these waters could be perceived by Russia as a threat to its submarine operations, especially given the deployment of its new Yasen-class nuclear-powered cruise missile fast attack submarines (SSGNs). These submarines are equipped with long-range strike capabilities, capable of targeting critical infrastructure from the Norwegian and Barents Seas. Given the silence of such an asset, which is specifically capable of overcoming the now reduced Western anti-submarine warfare (ASW) capabilities in the chokepoint, any non-acoustic sensors could be targeted. Their relevance is indeed destined to grow since climate change will significantly reduce sound propagation in the Greenland Sea, more than halving the current detection range of passive sonars. Notably, there have already been incidents with undersea cables equipped with sensors in the Arctic, as in the case of the mysterious disappearance of an infrastructure for maritime research and surveillance off the coast of Norway in November 2021. In that case, as with other instances of damage to undersea cables in Arctic waters, such as the Svalbard Undersea Cable System – SUCS -, several clues raise suspicions about Russia.

The current and growing importance of undersea intelligence gathering for Moscow is confirmed by the fact that Russia itself is moving on several fronts to collect underwater data and map the seabeds of interest. A recent article by “The Times” revealed the discovery of underwater sensors off the coast of Britain. According to the UK Ministry of Defence, these sensors were planted by Moscow to detect the movements of the four Vanguard-class submarines (SSBNs), which constitute the UK’s constant at-sea nuclear deterrent. Russia also collects data from the European seabed through the numerous operations of its “Research Fleet”, composed of oceanographic vessels capable of mapping the underwater environment through the use of advanced surveillance technology. Yantar-class oceanographic ships have recently been spotted in the Mediterranean and the Irish Sea. Therefore, the addition of sensors to European infrastructure must be contextualised in an underwater environment that is increasingly central to Moscow’s strategic considerations.

Innovation and technology to secure the depths

As shown, adding sensors to submarine cables could result in complications of various kinds. So, what are the alternatives available to protect European underwater infrastructure?

Acoustic sensing technology

A possible solution could be the adoption of “fibre optic sensing” and more specifically Distributed Acoustic Sensing (DAS) detection. This technology transforms existing fibre optic cables into large and densely sampled listening arrays without adding any external sensors to the infrastructure. It works by connecting specialised measurement equipment to the dry-end of a fibre (e.g. on shore), detecting nanometre-level movements in the fibres and converting these movements into strain measurements similar to acoustic pressure. In particular, laser light pulses are passed down the fibre and some of this light is reflected and scattered by imperfections that are naturally present on each fibre – a process known as “Rayleigh scattering”. Since these imperfections undergo tiny changes in position due to any changes in pressure and the surrounding environment, it is possible to precisely identify the location and source of interference on the cable.

In this sense, DAS detection has great potential in subsea defence activities by providing the ability to track both surface and subsurface targets and integrating its data into naval intelligence capabilities. A particularly interesting application concerns the possibility of reducing the dangers arising from anchor dragging, which has recently constituted the main threat to the safety of European underwater infrastructure. DAS detection would allow for the detection of sound waves from objects dragged on the seabed (e.g. anchors or trawls) at a distance of kilometres, which would usefully help alert authorities in advance. In this sense, it would also be possible to identify the interfering vessel by triangulating the data with those of the Automatic Identification System – AIS – on board. In addition to threats from surface ships, this technology could also respond to incoming threats from submerged vehicles such as submarines and uncrewed underwater vessels – UUVs. Indeed, although DAS appears to be optimised for detecting low-frequency emissions of higher intensity than those from submarines and UUVs, the sound signature of such vehicles could still be detected by fibre optic sensing but at a reduced range.

DAS detection has two features that make it advantageous compared to SMART Cables. First, it can be applied to existing infrastructures without adding sensors to them, using only a “dark” – unused – fibre instead. Second, unless the data were to be shared publicly for scientific reasons, no one outside the cable operator would be aware of the information gathering. These two factors would help overcome the two main obstacles explained above regarding the addition of sensors on cables. On the one hand, it would not be necessary to request specific permits from coastal states; on the other hand, the infrastructure would be less likely to end-up in the crosshairs of hostile actors interested in preserving the “opaque” nature of the underwater domain.

Despite these advantages, DAS is still a nascent technology, and detection capabilities are still limited by depth, rough seabeds, background noise and other environmental factors. Furthermore, some operators may still be reluctant to use their cables for this application, either because they would have to leave a fibre unused or because they may still fear the militarisation of the infrastructure. For these reasons, the possible use of fibre optic sensing cannot be the only solution but must be integrated and coordinated with other technologies capable of contributing to Underwater Domain Awareness – UDA.

Mobile platforms

A favourable complementary approach to DAS detection could be deploying sensors on mobile platforms such as Uncrewed Surface Vessels – USVs – and UUVs. Such naval drones can integrate hydroacoustic, magnetic and optical sensors to monitor seabed conditions in real-time and transmit data to Command and Control (C2) centres to be integrated with other naval intelligence sources such as satellites or fixed underwater sensors. In addition to being supported by academic research and tested on a multi-threat sabotage scenario, this approach has recently seen application with the Baltic Sentry mission. Launched in January 2025 by NATO allies and regional partners, the initiative seeks to bolster the monitoring of critical seabed infrastructure through a combination of manned and unmanned assets. In fact, the Baltic Sea, like other European marine areas such as the North Sea or the Mediterranean, given the relatively narrow and enclosed nature, presents particularly suitable features for the use of such platforms. The short distances to be covered help mitigate the problems of endurance and communication that still plague naval uncrewed platforms, especially UUVs. The same would apply to some waters affected by the routes of European underwater critical infrastructure, such as the Red Sea.

Mobile platforms offer four key advantages over fixed infrastructure-based sensors. First, they allow an avoidance of the legal and jurisdictional problems highlighted in the case of SMART cables. Deploying an unmanned vehicle over a cable route, even with active sonar, is deemed free navigation of the high seas and is considered a legitimate shallow or deep-water monitoring system necessary for submarine cable maintenance. Second, they reduce the likelihood of adversarial targeting, as they are harder to locate and disrupt than stationary monitoring points. Third, their mobility allows for adaptive surveillance, enabling operators to focus on specific areas where threats are detected or suspected, rather than relying solely on passive detection from static infrastructure. Fourth, they provide scalability and redundancy. In other words, multiple autonomous units can be deployed simultaneously to create overlapping layers of coverage, mitigating the impact of potential losses or malfunctions.

Conclusions

From what has been analysed, the coordinated and integrated adoption of DAS detection and sensors on mobile platforms appears to be a more effective and less risky solution than SMART cables. In this regard, it is possible to formulate some policy recommendations.

First, echoing the proposal of the EU Action Plan on Cable Security to launch a dedicated surveillance drone programme – air, surface and underwater –, we recommend that the EU continues to incentivise and support European projects focused on USV and UUV capabilities. Several ongoing programmes are already advancing these technologies for maritime security applications. One example is the Critical Seabed Infrastructure Protection (CSIP), which aims to ensure the protection of infrastructure from natural events, intentional attacks and deliberate sabotage through the use of both autonomous (AUVs) and remote-controlled (ROVs) underwater drones, as well as mobile and resident hosts. Another initiative worth highlighting is the recent SEabed and Anti-submarine warfare Capability through Unmanned featuRe for Europe (SEACURE), which promotes an integrated system of unmanned platforms to perform joint anti-submarine and seabed warfare operations to protect critical maritime infrastructure. Strengthening investments in these platforms would not only improve Europe’s ability to safeguard critical undersea infrastructure, but also enhance broader maritime situational awareness and deterrence capabilities. To counter the rising threat of hybrid attacks on underwater assets, EU defence initiatives should prioritise resilient, adaptable and scalable monitoring solutions.

Second, it is worth establishing a dialogue and a framework of cooperation with the submarine cable industry to promote the growing use of DAS detection and highlight the common benefits that it would bring. In this sense, the EU can count on some European champions such as the French Alcatel Submarine Networks (ASN), the world leader in the construction of submarine cables, and the Italian Sparkle, the fourth largest operator in the world for data traffic transported on the seabed. The adoption of DAS is already under consideration in some EU-funded cable projects, such as the MEDUSA cable and the Polar Connect, so the Union should push for the adoption of such solutions also on other projects and on existing cables.

Third, it is essential also to promote the integration of data collected by fibre optic sensing, mobile platforms and other intelligence sources such as satellites and surface vessels. This data can be fused and processed with the help of Artificial Intelligence (AI) and Machine Learning (ML) software in order to identify suspicious and potentially dangerous patterns of activity near underwater infrastructure. In January 2025, the multinational Joint Expeditionary Force (JEF) activated the Nordic Warden operation, centred around a UK-led and AI-based reaction system capable of assessing threats to undersea cables and monitoring movements of Russia’s shadow fleet. To activate similar operations in the EU, it is necessary to increase the support and financing of information-sharing platforms, such as the “Maritime Surveillance Network (MARSUR) III” or the recently proposed “Integrated Surveillance Mechanism for Submarine cables per sea basin”.

In conclusion, as recently demonstrated by the defence Readiness 2030 Plan, the EU is showing some signs of awakening on security issues. In this perspective, the “Action Plan on Cable Security” is certainly to be appreciated, especially in an often-underestimated domain such as the underwater one. Nevertheless, caution is needed. Certain decisions could hinder the resilience process and further endanger the infrastructure located on the European seabed. The Union must calibrate its response as quickly and effectively as possible, considering the political and military implications of the technologies chosen to secure their critical underwater infrastructure. Admittedly, any decisions will come at a cost and the EU must choose wisely how to proceed according to a trade-off between the effectiveness of the chosen measure/technology and the consequent risk of aggravating relations with Russia and lowering the threshold for possible escalation.

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