It's a stormy autumn day out in the Baltic Sea, 22 kilometers off the Polish coast. The surface of the water is turbulent; beneath it, at a depth of 40 meters, is a Russian Navy diving drone. The aim and mission of the unmanned mini-submarine are secret. It is no longer in its position: in Rostock, at the NATO headquarters “CTF Baltic”, the drone is already on the radar. Even their design and equipment have been identified - and all of this with infrastructure that was not actually intended for military reconnaissance.
This is a fictional scenario, but one that the defense industry and military are definitely working on. Because the Baltic Sea is criss-crossed by data cables. In addition to their actual purpose - to transmit Internet data - they have a second, surprising benefit: the fiber optic strands can act as sensors for what is happening in their surroundings. For science they open a window into the deep sea, for the military they are a surveillance instrument. The two can hardly be separated from each other.
“Interrogators” record every vibration in the sea
The process is called “Distributed Acoustic Sensing,” or DAS for short. It is based on the fact that even slight fluctuations in temperature and pressure or seismic and acoustic vibrations change how the light propagates in the glass fibers and is partially backscattered. At some junctions at the end of the cable routes, there are now inconspicuous metal boxes in the server cabinets. These are so-called interrogators. They meticulously detect the changes in the light signals and record them.
THIS is tried and tested routine for scientists at the GFZ, the Helmholtz Center for Georesearch in Potsdam. They are already experimenting with a more powerful variant of fiber optic cable sensors. With the so-called SMART cables, short for “Science Monitoring and Reliable Telecommunications”, the measuring instrument is not the fiber optic itself. Instead, special sensors for pressure, temperature and movement are lined up along the cable harness every 20 to 30 kilometers.
So far only monitoring near the coast
The SMART cables provide more accurate data than the DAS method. They also overcome its fundamental weakness: with DAS, the analysis of light signals usually only works up to the first repeater, the first signal amplifier on the cable harness seen from the measuring point, which is around 60 to 100 kilometers away. This is enough for the research and monitoring of coastal waters or waters closely enclosed by land such as the Baltic Sea. But Charlotte Krawczyk wants to research deeper ocean areas. “They are completely under-examined,” says the director of the geophysics department at the GFZ. Continuous measurements would be important there, for example in order to collect data for climate or flow models.
The “SAFAtor” research project, funded with 30 million euros, is intended to close this gap. Krawczyk and her colleagues are currently testing SMART sensors in several geophysically and seismologically interesting areas: off Sicily, off Istanbul and on the coast of Chile.
But compared to DAS, the SMART sensors have one disadvantage: They cannot be retrofitted to existing fiber optic cables. The technology is therefore only an option where cables are newly laid. But that's exactly what's happening with some older cable routes, says Krawczyk. “We have to take advantage of the moment when the telecommunications cable operators make their plans on how they will replace cables and perhaps also add routes that are still missing.” The researchers' idea: They want to join a commercial project and use research funding to upgrade the cable that was already planned to a SMART version.
Detect undersea drones with a sabotage mission
Jürgen Scraback makes no secret of the security policy and military importance of fiber optic cable sensors. He heads the “Maritime Domain” department at the European Defense Agency EDA. Data collection using DAS is a topic that the agency is constantly dealing with. And the military is also interested in the more powerful variant. “We are working very hard on these SMART cables,” explains Scraback. As part of the “Critical Seabed Infrastructure Protection” project, the agency is planning a demonstration event in November. Industrial consortia that use SMART cables would be represented there. “They will show how critical infrastructure can be protected,” says Scraback.
In plain language, it is about the detection and monitoring of undersea drones with sabotage missions, similar to the fictional scenario mentioned at the beginning. Or about the detection of ships that are supposed to secretly cut communication cables or pipelines with their anchors. As an officer and practical man, Scraback is cautious about evaluating the technology: “Until I see it, I don’t believe the industry that it works.”
Worry about false alarms
Theoretically, the potential of SMART technology is immense. There are now sensors that can detect explosives under water, says Scraback. But in practice there are still hurdles to overcome. The sensor processes produce huge amounts of data. Most of it is noise. The exciting signals can be distilled using artificial intelligence. But this AI has to be trained extensively. After that, computing capacity was also needed for operation. Whether the effort would be in a reasonable relationship to the benefit remains unclear for the time being.
“The underwater world is the most complex thing you can imagine,” says Scraback. What was trained into an AI model in one place no longer works the next. The change in salinity caused by ocean currents alone could confuse a warning system. “We are initially very skeptical as to whether this will trigger countless false alarms,” he says.
Can the cables also locate nuclear submarines?
Jonas Franken from the Chair of Science and Technology for Peace and Security at TU Darmstadt is also skeptical about what is perhaps the most speculative military aspect of sensor fiber optic cables. If they were deployed across the world's oceans at some point, wouldn't they be able to help pinpoint the position of nuclear submarines and thus undermine a core element in the balance of geopolitical-strategic mutual deterrence? “You have to start from a bit of empirical perspective,” says Franken. He does not believe that SMART cables will play a major military and security role compared to Distributed Acoustic Sensing. “Precisely because they cannot be retrofitted, but have to be built from scratch,” he says.
On the other hand, the security policy aspects of distributed acoustic sensing, i.e. the observation of water traffic near the coast, are clearer - especially now in times of hybrid threats, says Franken. Especially since DAS technology is currently being drilled out. The manufacturer Alcatel Submarine Networks, which is majority owned by the French state, is currently developing systems that can “see” and measure data beyond the first repeater or even further repeaters on the cable route - this significantly extends the range of the process.
Conflicts of interest between scientists and military personnel
In the EU-funded “Submerse” project, 24 partners – including the GFZ in Potsdam – are working on researching seismic signals and underwater noise using underwater cables. Rene Belso from the Danish digital infrastructure consortium “Deic” wrote a white paper for the Submerse project in 2025. It analyzes the constantly resonating questions of national security and the conflicts of interest between scientists and the military.
Research would be best served by “open data”, i.e. the open sharing of measurement data for scientific purposes. Military authorities, on the other hand, demanded that sensitive data be filtered out, from which, for example, the activities of their own ships or submarines could be read. “But the genie is out of the bottle, and it is definitely a dual-use technology,” says Belso, meaning a technology that can serve both civilian and military purposes. This in turn leads to sensor cable projects being financed for which there would have been no funding if the interest had only been purely scientific. "Something like this happens at the national level. I can confirm this for Denmark and Norway, and there is probably cooperation with the military in Sweden too," says Belso.
Researchers must keep their data confidential
This can be a tricky situation, because if a fiber optic cable equipped with sensors has a dual-use character, then in the event of a military conflict it could be a legitimate target under international law. Both Jonas Franken from TU Darmstadt and Jürgen Scroback from EDA point this out. The only question is: Would a cable that is demonstrably only used for communication purposes enjoy any protection status? And would combatants comply? For Rene Belso, smart or not smart makes no difference here: “It would be a target, like any critical infrastructure.”
The key decision in the SAFAtor research project is still to be made: Where should the planned smart test route run through the deep sea? “Of course, even if it is an infrastructure project, we are looking for places where we can also do scientifically exciting things,” says Charlotte Krawczyk. But choosing the route isn't enough. The companies that lay and operate the cables also come into play. “We first have to build trust because we don’t want to and are not allowed to intervene in the infrastructure and its performance,” says Krawczyk. Research must practically piggyback on commercial operations. “And that’s not necessarily trivial.” Before the practical test, a data standard would first have to be developed to deal with the expected flood of data and to make the data uniformly interpretable for researchers worldwide.
And of course Krawczyk and her colleagues are also aware of the dual-use nature of their research area. During joint development work with sensor manufacturers, they signed confidentiality clauses and accepted the requirement to publish data in reduced resolution or only after a certain period of time. That wouldn't change the basic scientific attitude of their work. Krawczyk says: “Something will definitely happen to science if everything is not completely free.” But this also applies to other data sets or other branches of science. The promise of cable sensors is to make previously rarely explored parts of the oceans scientifically accessible. The strict rules are the price that researchers have to pay in order to fill at least some of the data gaps in the vastness of the world's oceans.
