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The security of subsea cables: an enduring naval challenge

Subsea communications

Naval historian Andrew Boyd examines the strengths and vulnerabilities of subsea communication systems

The history of cable communications goes back to 1837 with Samuel Morse’s invention of the telegraphic cable. The first cross-Channel link was installed in 1851 and the first transatlantic one in 1858; by 1900 there was a vast global network of cables linking all major cities, commercial centres and important military sites.

A grapnel resting against the cable sheaves at the stern of the Great Eastern during the laying of the 1866 transatlantic telegraph. Grapnels were used to recover the ends of a broken cable, but in times of war also to raise and identify cables used by an enemy so they could be cut. Photo: PD

Once the imperial system was complete, the British planners focused on cable-cutting, to disrupt the communications of potential enemies in wartime. In 1912, the Committee of Imperial Defence approved plans for the Post Office, under Admiralty direction, to cut all Germany’s main international cables, if need be, to isolate it from the outside world. On the outbreak of war in August 1914 HMTS Alert cut German cables from Emden down the Channel to France, Spain, Africa and the Americas. Separate operations severed the cables to Britain, and those between Liberia and Brazil and to the Azores and New York.

Throughout the war the Germans too devoted significant effort and considerable ingenuity to cable-cutting. Initially, they focused on isolating Russia from its Western allies by severing cables in the Black Sea and the Baltic; in response Britain laid new cables to Russia, around northern Norway. From 1915, Germany conducted a sustained attack on cables connecting Britain with neutral countries across the North Sea, but Britain always retained sufficient control of the North Sea to repair damage there, suffering disruption rather than permanent loss. The Germans had initially avoided cutting Allied transatlantic cables, judging that this would harm its own communications as much as those of the Allies, but in the last year of the war, following the American entry, the Germans decided they would now gain more than they lost, and attempted to sever every cable. Most cutting was conducted by U-boats, a technique Germany had mastered over the previous two years. Although many cables were cut, the implementation fell short of the comprehensive and simultaneous impact sought – but the potential of the operation at a critical point of the war was clear.

The effect of HF radio

Midget submarine XE-4, which cut Japanese communications cables during the Second World War. Photo: PD

Unsurprisingly, then, in the early 1920s the Admiralty saw submarine attack on cables as a major potential threat. However, the development of High Frequency (HF), or shortwave, radio revolutionised global communications, transmitting not only traditional telegraphic text but also voice and high-speed teleprinter traffic over intercontinental distances. Between the late 1920s and the 1960s HF radio displaced cable as the preferred means of long-range communication; the first submarine telephone cable was not laid until 1956. Meanwhile, the invention of satellite communication in the mid-1960s suggested that the future would lie there rather than in any extensive return to cable.

In the Second World War the dominance of HF for long-distance, especially transoceanic, communication had meant less cable-cutting; in September 1939, however, the British did cut the German telegraphic cables from Emden to Lisbon, and from Emden to New York via the Azores. The Germans, meanwhile, assessed that British dominance in the Atlantic and Mediterranean and advances in cable repair techniques would render unproductive any attacks on the Allied cables there. In the Pacific and Indian oceans there was more action: the Imperial Japanese Navy attacked British cables and relay stations, sometimes by submarine, and the Royal Navy developed midget submarines for cable attack; at the very end of the war, the midget XE-4 cut the Japanese telegraphic cable linking Saigon to Singapore and Hong Kong. This operation aimed to force the Japanese to resort to radio communication, which was more vulnerable to interception and cryptographic attack.

The Cold War

The perceived vulnerability of radio, and the development of higher capacity cables, drove both sides to move their sensitive communications underground wherever they felt confident of geographic control. Both sides then looked for opportunities to gain covert access to their opponents’ communications by tapping into their cables. This interception resumed what Britain had done to great effect in the First World War, though with one key difference. In the earlier conflict the cable-cutting had forced Germany to use neutral cables which passed across British-controlled territory, and Britain had accordingly been able to demand copies of all traffic, without necessity for taps.

The first big tapping operations of the Cold War, conducted jointly by the British and Americans, attacked Soviet military cables in Vienna and Berlin by tunnelling from Western territory into Soviet-occupied zones. Then in the 1960s the Americans exploited their new submarine technology to attack maritime Soviet targets, and in 1971, they tapped the Soviet cable linking the naval base at Petropavlovsk on Kamchatka to the base at Magadan. This operation, codenamed Ivy Bells, delivered outstanding intelligence for ten years until it was betrayed. By that time the Americans had an even more valuable tap on the Barents Sea cable connecting Severodvinsk with the Soviet Northern Fleet headquarters at Murmansk.

Geography denied the Soviets similar opportunities. But they devoted considerable effort to planning how to attack and disable the American long-range passive sonar chains in the Atlantic and Pacific that were monitoring Soviet submarine movements. It can be assumed that the targeting of undersea cables for intelligence collection or military disruption by states with the relevant capabilities continues to this day.


Although over the 30 years from 1956 there were steady improvements in the international telephone network, there were physical limits to the development of traditional copper-based cable. Then in the 1980s came fibre-optic. The first of these cables laid across the Atlantic in 1988 could handle 40,000 simultaneous telephone calls, more than ten times the capacity of the best copper cable. The evolution of this technology was rapid; within a decade fibre-optic cable could handle millions of calls per minute and had supplanted satellite and residual HF radio. Fibre-optic made the internet possible, underpinned its explosive growth, and made cloud computing possible. It has rightly been described as the central nervous system of the global internet, revolutionising international trade and finance, where a typical day may now see ten trillion dollars in financial transfers. By 2022, it was estimated that 99 per cent of global communications used undersea cables, comprising a network of over 400 separate cable systems and 750,000 miles of fibre.

All international activity – business, political, military and personal – now depends on these networks. Furthermore, the integrated nature of the global communications infrastructure and the way traffic is routed means that any single communication probably uses an undersea cable at some point in its journey. National governments accordingly recognise that cables are a vital part of the critical national infrastructure enabling their country to function. However, while networks within a country can be protected by it, undersea cables invariably transit international waters, outside the control of any single jurisdiction, and their status is further complicated because most are owned by private companies. These present, therefore, a major vulnerability.

Threats and remedies

The Russian ‘research vessel’ Yantar, which, according to US Navy reports, is built to act as a mother ship to submersibles operating at depths up to 6,000 metres. The vessel has been spotted in close proximity to submarine cables in many parts of the world. Photo:

Undersea cables are routed, and strengthened where possible, to minimise the likelihood of accidental damage from fishing or anchoring, or from natural events such as earthquakes or landslides. The networks are designed to be resilient, with built-in redundancy and alternative routes, and rapid response teams to tackle repairs. Even though repairing a broken cable is still challenging and may take several weeks, damage from accidental human intervention or even a large natural event is generally manageable, inflicting inconvenience on users rather than critical loss.

The major threat, with potentially devastating consequences to the functioning of a modern economy and society, comes from attack by a hostile power or a terrorist organisation. Such an attack would target critical chokepoints, including cable landing stations where numerous cables come ashore, and/or simultaneously hit multiple cables offshore for maximum effect and an impact potentially comparable to an attack with weapons of mass destruction (WMD). Conceptually, it would replicate the putative 1918 German attack, but would exploit all the advantages of modern technology to achieve infinitely greater damage.

While a terrorist organisation might be willing to inflict unlimited harm, there would be limits to its resources and expertise, reducing the scope of what it could achieve, especially underwater. Good protection of vulnerable points such as landing stations, network redundancy, regular surveillance of critical cable routes, good intelligence, and an ability to respond rapidly to any perceived threat can reduce such a risk to acceptable levels; in the United Kingdom, threat monitoring and response is coordinated by the Joint Maritime Security Centre, with representation from all relevant agencies.

A state actor may, in contrast, have the resources to inflict huge damage but would have to carefully weigh the benefits and costs of an attack, as did Germany a century ago – but now, in a more globally integrated world, bringing consequences which are harder to predict. The mitigation factors against terrorism all apply to a state threat, but the WMD analogy suggests two other ways of increasing the difficulty and the risk of mounting such an attack: the first is to establish an international legal framework, backed by clear sanctions, to protect undersea cable communications and inhibit actions and capabilities required for an attack; and the second, the final resort, must be deterrence, in that an attacker must be convinced that the probability of a proportionate response is too high.

Dr Andrew Boyd CMG OBE is a former submariner and the author of British Naval Intelligence Through the Twentieth Century (Seaforth), winner of the Maritime Media Awards 2021 Mountbatten Award for Best Book.