Recently, there have been increasing reports, particularly in the Baltic Sea, of damage to submarine cables. In today’s technology-dominated world, perhaps not many people are surprised that cables can be found under the seas, but at the same time it is probably not clear to many what their purpose is. In this article, we aim to provide an overview of undersea cables, their vital role in the world, the reasons for the increased likelihood of their intentional damaging, and the complexities of investigating the recent Baltic Sea incidents.
What are undersea cables?
Probably few people think about how an email sent from Hungary can arrive anywhere in the world in a matter of seconds, or how you can have a video call with someone halfway around the world with almost no delay. The instant global communication we take for granted is made possible by an extensive network of undersea cables. Undersea cables are fibre optic cables laid on the ocean floor that enable data to travel between continents. They are the backbone of the global internet: more than 95 per cent of all the data that travels around the world – including calls, website data, trillions of dollars’ worth of financial transactions and sensitive government communications – passes through them. A single cable allows millions of people to watch videos or send messages simultaneously. The cables are capable of transmitting several terabits of data per second, making them the fastest and most reliable data transmission method available today.
Although Starlink (Elon Musk’s company) has promoted satellite internet, it is not a substitute for underwater cables. Satellites are a good solution for providing internet in remote, isolated locations or as a back-up in case of emergencies, but they cannot completely replace the physical infrastructure on the ground (and satellite systems rely on wires to connect to the internet too). In terms of capacity, there is no better technology than fibre optic cables, and they can carry much more data much cheaper than satellites.
Worldwide telecommunications began in 1850, when the first commercial international undersea cable was laid between England and France. By 1858, the first transatlantic telegraph cable connected London to North America (143 words were transmitted over it in about 10 hours at this time). There are now more than 600 active and planned undersea cables on Earth, with a total length of more than 1.48 million kilometres, roughly the diameter of the Sun.
Submarine cables were initially owned by telecommunications companies, which formed consortia of parties interested in using them. As the internet developed in the 1990s, more and more companies saw an opportunity in investing in the related infrastructure that allowed the internet to go global. In recent years, content providers such as Google, Meta and Amazon have also become prominent investors in the development and deployment of new cables. The capacity added by private companies to the overall network of submarine cables now far exceeds the growth attributable to traditional telecom operators.
Facts about the cables
At the bottom of the ocean, a cable is typically only as wide as a garden hose. Cables need to be thin and light to make them easier to pull up and repair when needed, as pulling up a large, heavy cable from thousands of metres below the surface would put a huge strain on the cable. Cables closer to the shoreline are usually covered to provide greater protection, as they are more likely to become entangled in fishing nets and anchors.
Undersea cables contain several optical fibres; these are glass or plastic fibres about the diameter of a human hair that use light signals to carry large amounts of data over huge distances with minimal loss. The intertwined fibres are encased in protective layers designed to withstand harsh subsea environments such as pressure, abrasion and, as far as possible, possible damage from fishing activities or ship anchors. According to the Dgtl Infra research team, a submarine cable costs about $64,000 per kilometre, and a new transatlantic cable costs $200-250 million.
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Damage and repair
Cables are exposed to extreme environmental events, from undersea volcanic eruptions to typhoons and floods. More commonly, currents rub the cables against rocks, damaging them over time. But 70-80 per cent of the damage is caused by accidental human activities, such as anchor setting or trawling (despite the fact that marine charts show cable protection zones). Such incidents tend to occur in depths of 200-300 m, but fishing is expanding to deeper waters (up to 1,500 m in the North-East Atlantic, for example). But as fishing and shipping become more sophisticated, avoiding cables may become easier. The advent of automatic identification systems in shipping has led to a reduction in anchoring damage, as a service is now available that allows vessels to slow down and anchor following a set pattern, avoiding cables. However, in areas of the world where fishing boats tend to be less sophisticated and operate with smaller crews, anchor damage continues to occur.
However, the global cable network is usually very resistant to damage. According to the International Cable Protection Committee (ICPC), the global network suffers between 150 and 200 incidents of damage every year, which is not much compared to the nearly 1.5 million kilometres of cable. In addition, in most cases, when damage does occur, it can be repaired relatively quickly. The ICPC was founded in 1958 and its membership consists of government administrative bodies and companies that own or operate submarine telecommunications or power cables, as well as companies with an interest in the submarine cable industry, including most of the world’s major cable system owners and cable ship operators. The ICPC’s primary objective is to assist its members in improving the safety of submarine cables by providing a forum for the exchange of relevant technical, legal and environmental information.
You rarely hear about cable failures because most companies that use cables divide their network capacity across multiple cables, so if one fails, the network will run smoothly on the others until service is restored on the damaged cable. This is known as redundancy, and Google, Facebook and Microsoft have spent hundreds of millions of dollars on their own undersea internet cables in recent years because of the need for it. When a cable fails, a ship is sent to repair it. There are twenty-something such ships around the world, strategically stationed in locations from where they can get to a failed cable as quickly as possible. A deepwater repair typically takes one to two weeks, depending on the location and weather. A deepwater repair typically takes one to two weeks, depending on the location and weather. This doesn’t mean that an entire country’s internet connection is down for that long: many countries have more cables and more bandwidth than the minimum required within those cables, so if some cables are damaged, the others can take the load.
However, some damage is inevitable. The eruption of the Hunga Tonga-Hunga Ha’apai volcano in 2021-2022 destroyed the undersea internet cable connecting the Pacific island nation of Tonga to the rest of the world. It took five weeks to get the country’s internet connection fully operational again, although some temporary services were restored after a week. Although this massive volcanic eruption was an unusually large event, the cable connection of an island nation in a volcanically active area always involves some risk.
Significant routes
The Euro-Atlantic is the oldest and one of the busiest submarine cable routes. The routes connecting Europe with Asia (via the Mediterranean, the Suez Canal and the Red Sea) and Asia with the United States (via the Pacific Ocean) are also busy. Arctic routes from Europe to Asia are increasingly being exploited as they cover much shorter distances, but Arctic cables face significant technical challenges and are not yet a viable alternative route.
The section of the route connecting Europe to Asia that goes through the Mediterranean, the Suez Canal and the Red Sea – which includes a 160-kilometre stretch of land in Egypt – is probably the most vulnerable part of the internet globally, due to the concentration of internet cables here. A June 2022 European Parliament report also highlighted the risk of widespread internet disruption in the region. However, in addition to cost efficiency – this route is the shortest (mostly) undersea route between Asia and Europe –, the Red Sea cable concentration has the added advantage of interconnectivity.
The global submarine cable system. Source: https://www.submarinecablemap.com/
Increased vulnerability
Following the explosion of the Nord Stream gas pipeline in September 2022, governments around the world have pledged to better protect underwater infrastructure and subsea cables. Several incidents have since highlighted this vulnerability: in March 2024, for example, several cables failed off the coast of West Africa, leading to major internet disruptions in at least ten countries. The cause was almost certainly an underwater mudslide or avalanche, which are quite common in the region. A few weeks earlier, a ship’s anchor snapped cables serving the East African and Asian coasts in the Red Sea after the vessel was hit by a missile fired by Yemeni Houthi militants.
A US cybersecurity firm, Recorded Future claimed in a 2023 report that Russia is closely monitoring undersea cable systems, most likely for potential sabotage in retaliation for Western support of Ukraine. The United Kingdom claims that Russian submarines are monitoring cables coming into the country. The New York Times reported as early as 2015 that Russian submarines and spy ships were “aggressively” operating near undersea cables from the North Sea to Northeast Asia.
According to a 2024 report by Taiwan’s National Audit Office, foreign ships have damaged cables connecting the country to its outer islands 36 times since 2019, with 12 incidents registered last year. It cannot be ruled out that at least some of these incidents are the result of deliberate Chinese operations. In February 2023, two cables connecting Taiwan to the Matsu Islands were damaged within a few days by both a Chinese fishing boat and a Chinese cargo ship, causing slower internet connections and disconnected phone calls. A US national security expert described the incidents in Foreign Policy as a dry run for Taiwan’s “invisible blockade”.
Hotspots: the Baltic Sea and Taiwan
However, it is very difficult to determine whether a cable has been damaged accidentally or as a result of deliberate action, but geopolitical tensions are increasingly raising the suspicion of sabotage. This is particularly true in the Baltic Sea, where there have been several recent incidents and where dozens of internet and energy cables are located, most of which are lying unprotected on the seabed. In one of the most recent such incidents, in January this year, Sweden seized a ship on suspicion of sabotage after a communications cable linking Sweden and Latvia under the Baltic Sea was suspected of being damaged. Russia denies any involvement in the incident, but NATO says many of the ships behaving “strangely” in the Baltic Sea are from or heading to Russian ports. Also in January 2025, the British navy escorted an alleged Russian spy ship through the English Channel, weeks after it was caught “hovering over critical subsea infrastructure” in UK waters.
On 25 December 2024, the Estlink 2 electricity cable linking Estonia to Finland failed, reducing the amount of electricity reaching Estonia by almost two-thirds. The next day, the Finnish authorities detained a Cook Islands-flagged tanker believed to be carrying oil from Russia to Turkey. The vessel is suspected to have passed over the cable while apparently dragging its anchor behind it. NATO, which had already been monitoring cases of suspected cable-cutting, reacted immediately, setting up a Baltic naval task force in three weeks specifically to deter such suspicious attacks. Announcing the new monitoring and deterrence mission, Baltic Sentry, NATO Secretary General Mark Rutte said he was “deeply concerned about the growing threat to our critical undersea infrastructure.”
While NATO’s heightened alert is mainly aimed at the “shadow fleet” of Russian smuggling vessels, Chinese-owned vessels have also been under suspicion, including in November 2024 when a cargo ship was detained for weeks in Danish waters after two fibre-optic cables were damaged. The ship, Yi Peng 3, after leaving a Russian port, allegedly damaged cables between Sweden and Lithuania as well as Finland and Germany. The vessel continued its voyage after investigators from Sweden and other countries were allowed on board. The Swedish authorities have not said whether they have found any evidence. China has said it will continue to cooperate with regional authorities in the investigation.
The area around Taiwan is also a hotspot for submarine cables. In early January this year, Taiwan’s coastguard detained a Hong Kong-owned cargo ship after Taiwan’s largest telecommunications company had notified the authorities that a submarine cable had been damaged in the area on 3 January. The preliminary assessment of the situation is that the damage may have been caused by the cargo ship; the Taiwanese government believes that it may have been a Chinese “grey zone intervention” (i.e. an action designed to wear down an adversary without actually engaging in a firefight). The incident did not cause any disruption to Taiwanese communications as data was being transmitted on other cables.
Sabotage or accident?
Although European authorities have detained ships in cases of suspected sabotage, they have not directly accused Moscow or Beijing in the absence of concrete evidence. Deliberate damage is extremely difficult to prove, and the complexity of what may at first seem to be obvious cases is illustrated by the fact that, according to press reports, damage to energy and communication lines in the Baltic Sea in recent months has likely been the result of accidents rather than Russian sabotage, according to several US and European intelligence officials. The emerging consensus among US and European security services is that the evidence gathered so far suggests that the accidents were caused by inexperienced crews serving on poorly maintained vessels.
Not everyone accepts this explanation (including Finland’s former chief military intelligence officer), citing anomalies in the behaviour of the ships involved and evidence that Russia has devoted significant resources to mapping and identifying vulnerabilities in the Western seabed infrastructure for decades. At the same time, one might reasonably ask why Russia would risk targeting submarine systems in waters bordering NATO member states. This would jeopardise the oil smuggling that Moscow needs to finance the war in Ukraine, and could potentially lead Western governments to make greater efforts to block Russia’s shipping lanes to the North Atlantic.
Summary
Without undersea cables, the world would be unimaginable. Despite their total length being equivalent to the diameter of the Sun, damage is rare and usually repaired quickly. But the possibility of sabotage is a growing concern. Cables are often located in isolated but publicly known locations, making them an easy target for enemy action. Complicating matters, intentional damage is extremely difficult to prove beyond reasonable doubt. In the current context of great power disputes and geopolitical tensions, it is therefore to be expected that deliberate damage to cables will become more frequent – however, as some recent incidents in the Baltic Sea have shown, what initially appear to be clear cases of sabotage may turn out to be the result of accidents.
