Beneath the icy surface of the North Sea, a diver spotted a fibre optic cable stretched out along the seabed — one of the many arteries powering our connected world. As he drew closer and reached out a hand to touch it, he had no idea he was already being monitored.

“He stops and just touches the cable lightly — and you clearly see the signal,” says Daniel Gerwig, global sales manager at German tech firm AP Sensing. “The acoustic energy traveling through the fibre disturbs our signal. We can measure that disturbance.”

It's a powerful reminder that today’s fibre broadband infrastructure isn’t just a highway for data — it’s also a frontline sensor in an increasingly complex world.

In recent months, multiple incidents of damaged telecommunications cables in the Baltic Sea have raised alarm across Europe. These undersea cables are critical to the delivery of high-speed internet and cloud services, like those offered by Telecoms World Fibre Broadband, forming the invisible scaffolding of our digital lives.

Recognizing the strategic importance of this infrastructure, NATO has launched a new mission, Baltic Sentry, deploying drones, ships, and aircraft to patrol vulnerable areas. The EU is also expanding external surveillance efforts to protect the networks we rely on every day.

But as Gerwig notes, patrols can't be everywhere at once — and that’s where smart cable sensing technology comes in.

Cables That Listen

Companies like AP Sensing and Optics11 are pioneering ways to transform fibre optic cables into real-time listening devices. During a test last year — simulating, not sabotaging — AP Sensing tracked a diver’s movements as he gently patted a cable on the seabed. The system also picked up the approach of ships, drones, and even the impact of a plunging anchor.

This is possible thanks to how fibre optics work. When pulses of light travel through the cable, small reflections bounce back — and those reflections are subtly changed by things like heat, vibrations, or movement. A change in temperature could even signal that a buried cable has become exposed.

In one striking demonstration, the company buried a fibre optic cable near a lawn where a man walked and fired a rifle. Every footstep and the gunshot were visible in the cable’s acoustic signal — allowing analysts to identify not just the presence of a threat, but its size, movement, and direction. These data points can be paired with satellite images or ship tracking systems for a more complete picture.

For telecoms providers like us at Telecoms World, the potential of this technology is game-changing. Not only could it be added to existing networks — using “dark” fibre or spare capacity — but it could also help us offer an even more resilient, secure service to our customers.

Challenges Beneath the Surface

Of course, no system is perfect. Signal listening devices — or “interrogators” — need to be placed approximately every 100km. While some disturbances can be detected from hundreds of metres away, longer-range detection is still limited.

And physical cable protection has its limits too. Today’s subsea fibre cables are wrapped in tough metal sheaths and reinforced with “armoury wire” — often in double layers for especially hazardous environments like the UK’s rocky seabeds. Still, a ship dragging a heavy anchor can cause major damage, even to double-armoured lines.

Burying cables deeper into the seabed adds extra protection, but doing so over long distances is often prohibitively expensive.

Always On, Always Redundant

At Telecoms World Fibre Broadband, we understand that network reliability is non-negotiable. That’s why redundancy is built into our systems — meaning that even if a cable is cut, your connection stays live through backup routes. As industry analyst Lane Burdette from TeleGeography points out, the global average of 100–200 undersea cable faults a year has remained steady, and users rarely notice a blip thanks to this built-in resilience.

But as the global climate grows more tense, awareness and preparedness are more vital than ever. The growing use of fibre acoustic sensing, combined with increased military presence in the Baltic and other key regions, signals a shift in how we protect the critical infrastructure underpinning the internet.

As Thorsten Benner of the Global Public Policy Institute puts it, “It’s good that NATO and the EU have woken up.”

The true measure of this technology’s effectiveness lies in what happens after detection — in how quickly an alert reaches those who can act, and how swiftly they respond.

Until then, our fibre cables — the very ones powering your streaming, gaming, video calls, and work-from-home sessions — are quietly listening beneath the waves, keeping the world more connected and more secure.

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