New fibre optic sensing technology is being deployed under the Baltic Sea to detect and deter sabotage attempts on Europe’s critical subsea data cables.
Subsea Cables Carry the World’s Internet
Many people may not know that nearly all global internet traffic depends on cables running across the seabed. Although these are slender fibre optic lines, often just a few centimetres thick, they actually carry around 99 per cent of intercontinental data! As such, these cables link whole countries, power financial markets, and support global communications infrastructure.
For example, the seabed beneath the Baltic Sea is criss-crossed by dozens of these connections, linking Sweden, Finland, Estonia, Latvia, Lithuania, Poland and Germany. These cables are essential to both internet access and the flow of electricity from offshore renewables.
However, these undersea cables are exposed and vulnerable to damage, e.g. from anchors or fishing gear, or even from deliberate sabotage, as tensions rise in the region.
Unexplained Cable Damage Has Triggered a Response
Several recent incidents have put the vulnerability of the subsea cables in the Baltic firmly in the spotlight. In late 2023 and early 2024, multiple data and power cables were damaged in unexplained circumstances. Although investigations are still ongoing, European leaders have taken the incidents seriously.
The EU’s New Initiative To Protect The Cables
In February 2025, the European Commission launched a new initiative to protect subsea infrastructure, warning that cable disruptions “risk causing severe interruptions to essential services” across the EU. The strategy sets out new funding, detection systems, mapping efforts and plans for smarter, more secure cables.
The NATO-led “Baltic Sentry” mission, using drones, warships and aircraft to patrol the sea, was also launched in response. However, with thousands of kilometres of seabed to monitor, authorities have found it necessary to turn to a new kind of defence, i.e. smart sensor cables that can detect threats in real-time.
How Fibre Optic Cables Can ‘Hear’ Underwater Activity
The idea is deceptively simple, i.e. use existing fibre optic cables to listen for disturbances. This is made possible by a technique called Distributed Acoustic Sensing (DAS). In short, light pulses are sent down a fibre optic line. When that line is disturbed (by vibrations, movement or temperature changes) it alters the light signal, which can then be analysed.
Examples of what these fibre optic sensor cables can detect include:
– Divers, drones or submersibles near the cable.
– Anchors being dropped or dragged along the seabed.
– Vessels passing overhead, including their size, speed and direction.
– Dredging, tunnelling or unauthorised seabed activity.
During tests, it’s been found that such acoustic sensing systems could detect a diver simply brushing the cable. In one demonstration, a buried cable was even able to pick up nearby footsteps and even a distant gunshot.
Cross-Referenced
The information from the cable can be cross-referenced with satellite images or vessel tracking data to confirm whether suspicious activity is occurring. In practice, it becomes a silent surveillance network lying on the seabed, detecting threats without giving away its presence.
Who’s Building These Systems And Where Are They Being Used?
Several companies are already rolling out this technology, with Germany’s AP Sensing and the Netherlands-based Optics11 leading the charge.
AP Sensing’s solution uses Distributed Fibre Optic Sensing (DFOS) to monitor telecoms cables, power lines and gas pipelines. The company says its system can provide real-time alerts and pinpoint exactly where a disturbance is occurring, even differentiating between routine vessel traffic and potential intrusions.
Although AP Sensing has confirmed that its systems are in place on some cable routes in the North Sea, it has not disclosed specific locations for security reasons. However, the Baltic is a clear focus area for future deployments.
Optics11 is developing systems that can be laid independently of the main cables. The company says these could act as early-warning sensors in strategic zones, e.g. 100km from a critical port or around key gas pipelines.
How The Sensors Are Deployed
Both AP Sensing and Optics11 say that these sensor systems can be deployed without laying new cables. Instead, they rely on something called “dark fibre” which are unused strands within existing fibre optic cables that were installed for future capacity. These dormant fibres can be activated and connected to specialist monitoring equipment on land, turning them into sensing lines without disturbing the cable itself.
If no dark fibre is available, the system can (sometimes) use spare channels on active fibres. In either case, the main undersea cables stay where they are, i.e. there’s no need to slide anything new into the seabed. It’s the light signals running through the fibre that are doing the ‘listening’.
A New Line of Defence – But It Has Limits
While the technology appears to be powerful, it isn’t perfect. For example, one of the key limitations is range. In most cases, disturbances must occur within a few hundred metres of the fibre to be reliably detected, so interrogators (the specialist devices that analyse the signals) must be installed roughly every 100km to keep the system functioning. This, unfortunately, makes full coverage expensive, especially across long distances or in deep water.
Another challenge is what happens after a disturbance is detected. For example, detecting a diver or anchor is useful but without a rapid response capability, there’s still a risk of damage before any intervention can take place.
Also, even the most physically robust cables are not immune. According to Swedish cable manufacturer Hexatronic, although lines are reinforced with metal armouring and thick protective layers, even these can be severed by a deliberate dragging of a ship’s anchor, especially at shallow depths.
Why the Baltic Sea Is Europe’s Top Priority Right Now
The Baltic region has become a focal point for infrastructure protection, and not by coincidence. For example, its shallow waters, heavy commercial traffic and its strategic proximity to Russia all make the area particularly vulnerable. Many of the recent cable damage incidents (and subsequent investigations) have occurred here, prompting coordinated action. In response, the European Commission has laid out a wide-ranging plan that includes:
– Mapping all subsea cables across EU waters by the end of 2025.
– Launching a “Cable Security Toolbox” of protective measures.
– Funding new smart cables with inbuilt sensing capabilities.
– Creating a list of critical “Cable Projects of European Interest”.
– Increasing repair capacity and backup systems to minimise downtime.
These efforts are being coordinated with NATO activities and national security strategies. The EU’s 2025 roadmap also links in with the NIS2 cybersecurity directive and the Critical Entities Resilience (CER) directive, two major legal frameworks designed to harden infrastructure against hybrid threats.
China Introduces a Powerful Deep-Sea Cable Cutter
Whilst this is all happening, China has unveiled a powerful deep-sea cable cutter capable of severing heavily protected undersea lines at depths up to 4,000 meters—beyond existing operational ranges.
Developed by the China Ship Scientific Research Centre and the State Key Laboratory of Deep-sea Manned Vehicles, it can be fitted to both crewed and uncrewed submersibles like Fendouzhe and Haidou. Originally intended for civilian salvage and seabed mining, its dual-use potential could alarm nations reliant on undersea communication cables that carry 95 percent of global data.
This is the first official disclosure of a device that could disrupt critical maritime infrastructure.
What Does This Mean For Your Business?
With geopolitical tensions rising and critical infrastructure increasingly targeted, fibre optic sensing offers a promising (and highly practical) way to spot trouble early. By transforming existing cables into silent surveillance tools, Europe may be beginning to close a dangerous blind spot beneath the waves.
However, as with most emerging technologies, this isn’t a silver bullet. Sensor systems still rely on swift responses from coastguards or military forces to intervene if sabotage is suspected. Their coverage is limited by distance, their deployment by cost. Even the most advanced system can only alert, it can’t physically stop damage from being done in the first place. That said, the strength of this technology lies in its ability to provide that vital early warning, buying time when it matters most.
For the UK, this move towards smarter cable monitoring is not just a continental concern. For example, Britain’s own subsea networks connect it to Europe, North America and beyond, underpinning the data economy, financial services and everyday digital life. UK telecoms providers and energy firms will likely need to follow developments in the Baltic closely, both to understand the risks and to assess the potential of retrofitting their own infrastructure with acoustic sensing capabilities.
At the same time, the opportunity for UK-based firms specialising in cyber-physical systems, marine engineering or signal analytics could be significant. As the EU ramps up investment in cable resilience and NATO deepens its interest in subsea surveillance, demand for this kind of expertise will grow across both civil and defence sectors.
The wider lesson from the Baltic, therefore, is that the digital world doesn’t float in the cloud but it sits on the seabed. As governments, companies and regulators begin to grasp just how critical and exposed that infrastructure really is, technologies like fibre optic sensing are likely to become part of a much broader push to harden Europe’s digital backbone. The challenge now is ensuring that detection leads to action, and that security keeps pace with the threat.