Defence technology

Internet cables on the seabed reveal ghost ships

Armed with satellites, radars, and drones, it is possible to detect ships engaged in espionage, attacks on critical infrastructure, or violations of Danish sovereignty. Now, researchers have discovered a new method for locating so-called dark ships—by using fibre-optic cables on the seabed.

In November 2024, two data cables in the Baltic Sea were damaged and the Chinese freight ship Yi Peng 3 was suspected of being the culprit by dragging its anchor along the seabed. Researchers at DTU hope to prevent sabotage on critical infrastructure by better detecting so-called dark ships. Photo: Mikkel Berg Pedersen/Ritzau Scanpix

Tuesday 28 October 2025

Sole Bugge Møller

Every day, millions of vessels traverse the world’s oceans—container ships, fishing boats, ferries, tankers, warships, and everything in between.

To prevent collisions, ships are legally required to transmit signals via an Automatic Identification System (AIS). Authorities also use AIS signals to monitor fishing activity, passenger ferries, sovereignty violations, and rescue operations.

Nevertheless, many ships deactivate their AIS signals. They are then referred to as dark ships. American research published in 2024 in the scientific journal Nature found that up to 30 per cent of the world’s vessels emit no AIS signal, prompting researchers at DTU to develop methods for identifying these ghost ships.

“Not all dark ships are engaged in illegal activity, but nearly all ships involved in unlawful acts are dark ships,” says Kristian Aalling Sørensen, postdoc at DTU Space, specializing in maritime surveillance and providing research-based support to authorities.

Such activities range from illegal fishing to drug and arms smuggling, espionage, and attacks on critical infrastructure. Hence, authorities need to be able to detect these ships swiftly and accurately.

“We naturally wish to prevent such actions, but the military does not have sufficient vessels to be everywhere. Surveillance of dark ships is one of our most effective tools, as it serves as a deterrent—the better our surveillance, the stronger the deterrence,” Kristian Aalling Sørensen explains.

Disguised as accidents

Alexander With, a military analyst at the Royal Danish Defence College who researches maritime hybrid warfare, notes a significant rise in incidents involving sabotage of maritime infrastructure since the war in Ukraine.

“There were also cases of damaged subsea cables and pipelines under suspicious circumstances prior to the war, but the scale has increased dramatically,” he says.

While Alexander With sees Russia as the primary threat in Danish waters, there have also been incidents involving Chinese vessels exhibiting suspicious behaviour near cables shortly before the cables were damaged. Typically, commercial ships drag their anchors across the seabed to damage cables and pipelines.

“That way it can be claimed to be an accident,” Alexander With says.

By sailing close to maritime infrastructure, ships may also gather intelligence on offshore wind farms, subsea pipelines, and fibre-optic cables—information that, according to With, could be used to plant explosives, as seen in the 2022 Nord Stream pipeline attack.

Facts

About Kristian Aalling Sørensen

Does research on dark ships at DTU Space
Cofounder of DTU startup Tordenskjold ApS, which makes software to identify dark ships
Satellite specialist in the Danish Army
Is the first Dane ever to receive the NATO STO Early Career Award for his research
On Danish newspaper Berlingske's renowned list Talent 100 in 2025

Detecting vibrations from ships

Until now, dark ships have primarily been monitored using satellites, coastal radars, and drones. However, Kristian Aalling Sørensen and his colleagues are among the first globally to employ a new method—using fibre-optic cables on the seabed for telecommunications.

When a ship passes above a fibre-optic cable, it sends vibrations through the water, which can be detected by the cable on the seabed.

Researchers place a small device at the end of the cable on land, which emits an electromagnetic pulse through the cable. This pulse can detect the ship’s vibrations and even be converted into sound, allowing researchers to estimate the vessel’s size—distinguishing between a small fishing boat and a large warship.

As the cable is used solely for transmitting electromagnetic pulses, the researchers cannot intercept any communications carried via the internet cables.

So far, the method has only been tested on a single cable from the Faroe Islands, but in principle, it could be extended to the many fibre-optic cables criss-crossing the seabed.

Immediate detection

The advantage of this method is its immediacy—researchers know exactly when a ship crosses the cable. In contrast, satellite imagery may take hours or days to capture a specific area.

“This delay is a major issue, as a ship may already have caused damage to critical infrastructure. Fibre-optic cables allow us to detect a vessel the instant it passes above, and we receive data at the speed of light,” Kristian Aalling Sørensen says.

The drawback is that detection is limited to the area directly above the cable. However, in narrow straits, cables often stretch from coast to coast and can function as an alarm system, detecting all dark ships passing, for example, Bornholm, the Øresund, or the Kattegat.

Another limitation is that the signal weakens with distance from land, and analyzing data from the cable is complex.

“There is a great deal of noise in the signal, making analysis extremely challenging due to its abstract nature,” Kristian Aalling Sørensen explains.

A more fine-meshed net

Fibre-optic cables alone cannot identify invisible ships. Still, when used in conjunction with satellites, radars, and drones, they become a valuable tool that tightens the net around dark ships. By compiling multiple clues, they can eventually determine why a dark ship is not transmitting an AIS signal. Is there a benign explanation, or is it suspicious that, for instance, a Russian container ship is suddenly near Greenland’s coast close to subsea cables?

“We have many surveillance technologies, and it is crucial to incorporate as many as possible, as each provides different insights. Now we have another tool in our toolbox,” Kristian Aalling Sørensen says.

Although the threat from dark ships has increased, Russia could easily escalate this hybrid warfare, Alexander With warns.

“Russia has not yet utilized its full capacity. They have deployed spy ships to map subsea infrastructure and know precisely where everything is located. If they chose to, they could wreak havoc on the maritime infrastructure that Denmark relies on,” he says.

But surveillance does make a difference, Alexander With asserts.

“If we don’t monitor dark ships, they can more easily drag anchors across the seabed and sever infrastructure without consequence. When we monitor them, we can see what they are targeting—and then assess whether that infrastructure is adequately protected,” he concludes.

Contact

Kristian Aalling Sørensen Postdoc National Space Institute Mobile: +45 31419971 kaaso@dtu.dk