Submarine cable resilience

The Matsu Islands are an archipelago of 36 islands in the East China Sea, alongside the southeastern coast of mainland China. The islands have been administered since 1949 by Taiwan, part of a larger set of outlying islands administered by Taiwan, which includes the Kinmen and Penghu Islands, which all lie in the Taiwan Strait.

The Matsu Islands are connected to Taiwan’s main island by two submarine cables — Taima No.2, Taima No.3, and soon Taima No.4. The Taiwan Strait is a shallow body of water, with an average depth of 70m. The submarine cables that lie on this seafloor are vulnerable to damage from ship-based activity, like being snagged by fishing equipment or anchors. But these particular cables have been remarkably unlucky. Between 2018 and 2022, the submarine cables between Taiwan and the Matsu Islands were cut 27 times, and there have been four major outages in the three years from 2023 to 2025.

That’s not the only location where submarine cables have suffered many faults. The Baltic Sea is a shallow sea that has a dozen or so submarine communications cables, and undersea electricity and gas pipelines. There have been 11 disruptions to these submarine systems since 2023. Again, a very unlucky track record.

There are an estimated total of 1.5M kilometres of subsea communications cables, used by 607 in-service submarine cable systems (Figure 1) that are critical to the world’s telecommunications services. If they are put out of service for any reason, our satellite systems simply could not replace them. In such a catastrophic scenario, some communities would be totally isolated, with severe logistical and economic ramifications.

We regard this collection of submarine cable systems as critical infrastructure for global communication services, and we take care to make them as resilient as possible. Modern warfare can deliberately disrupt communications systems, which adds another source of potential damage to the list of geological and accidental risks facing subsea cables.

On the night of 4 August 1914, days after Britain declared war against Germany, the British cable ship HMTS Alert sailed from Dover into the English Channel. The vessel dragged grappling hooks along the seabed, hauled up five German transatlantic and European telegraph cables, and cut them. This severed Germany’s direct, secure submarine cable links to the Americas, Africa, Spain, and the rest of Europe. Germany was forced to rely on wireless radio transmissions, where British intelligence could readily eavesdrop. If the German cryptography used to protect these wireless communications was broken, then it allowed the British to track military movements and capture critical messages, such as the Zimmermann Telegram.

This same action to deliberately sever undersea cables was repeated by the British in September 1939, cutting German cables to Lisbon and New York.

These thin polyester-coated cables at the bottom of the ocean are vulnerable, including to:

How can cables defend against such risks? The inner core of the fibre optic cables and its copper conductor sheath can be wrapped in additional layers of steel cable jacketing as a form of armour (Figure 2). This is mostly used to reduce its vulnerability to shipping accidents.

Figure 2 is somewhat misleading. What is depicted here is a cross-section of submarine cable used in the segments of the cable that lead to the surface cable station. The cable segments that are laid on the deep-sea floor use just the inner components: Optical fibre and a single thin steel thread wrapped in a silicone gel, surrounded by a copper sheath (used to power the cable repeaters), and wrapped in polyethylene. There are no external armour elements. These deep-sea cable segments are around 20mm in diameter, or the size of a garden hose.

The cable surfaces at a cable landing station. These landing stations are typically plain, secured facilities with reliable power. The preferred location is a coastal point with a low volume of fishing or other maritime traffic, and a steep rise from the deep-sea floor. Cables are at their greatest risk from coastal shipping activity, so the shorter the distance from the deep-sea floor to the cable station on dry land, the better.

Many jurisdictions also declare ‘Cable Protection Zones’ as a warning to marine vessel operators that there are cables on the seafloor in this area, and ships should refrain from fishing activity and anchoring in these areas.

Cable routing is also important in terms of resilience. There are several objectives in a cable route, and not all of them can be optimized at the same time.

The cable route should be as short as possible, as additional cable length raises capital costs and increases end-to-end signal delay.

Cables should avoid areas of active geological activity where possible, such as tectonic plate boundaries and mid-plate rifts. Deeper is better, but within reason. Cables on the floor of a deep ocean section are less vulnerable to shipping activity and hostile efforts to disrupt the cable, but with extreme depths it’s more challenging to locate and raise the cable when undertaking a repair.

Sometimes there are geographic ‘choke points’ through which many cables are routed. The Bab al-Mandab Strait is not just a choke point for surface vessels passing between the Indian Ocean and the Red Sea, but it is also the conduit through which 16 cables pass (Figure 4). In September 2025, several cables were severed in the Bab al-Mandab Strait, disrupting an estimated 25% of cable capacity between Asia, Europe, and the Middle East. Houthi-controlled waters remain effectively ungovernable for cable protection.

There is also the Singapore Strait, a major junction for 35 cable systems (Figure 5). This is a shallow water strait that also hosts a significant volume of surface shipping.

Further north, between Taiwan and the Philippines is the Luzon Strait, where 18 cables pass (Figure 6). This was the location of an earthquake and a subsequent set of submarine landslides in December 2006 that not only severed all the cables running through this strait, but also buried many of them under deep layers of sediment. It took 11 repair vessels 49 days to bring all the cable systems back into operation.

The Black Sea is an area with geopolitical risk and regulatory gaps. While mine risk is confirmed, no cable faults have been directly documented in the war in Ukraine. Russia’s continued presence and the conflict make this region functionally inaccessible for routine cable maintenance.

Geopolitics also plays a role in cable routing. Cable owners will prefer to avoid routes through politically unstable territorial waters if suitable alternative routes exist. There are also political considerations. If a cable lands in US territory, then the cable is subject to several Federal Communications Commission (FCC) constraints, including banning equipment or services from companies the FCC see as a threat to US national security. The FCC has also refused to permit the landing of a cable on US territory if the cable also lands in a territory that the US deems to be a foreign adversary.

Land routes are avoided in general, but there are some circumstances where a land route is the preferred option. Connectivity between Asia and Europe would normally run through the Bab al-Mandab Strait, landing in Egypt in the Gulf of Suez, then overland through Egypt to get to the Mediterranean Sea. There is an alternative route through the Persian Gulf, Iraq and Türkiye, although this entails a segment that transits the Strait of Hormuz, or a second overland segment through Oman and the United Arab Emirates.

Around 200 submarine cable faults are reported worldwide each year, a figure that the International Cable Protection Committee notes has stayed broadly steady in recent years even as the total cable inventory increases. The cables themselves are remarkably reliable. It is the maritime traffic above them that is the cause of most of the problems. 86% of cable faults are likely due to fishing activity or anchoring, and fishing activity is around twice as prevalent as anchoring incidents.

Geological events, such as submarine landslides and seabed movement, account for about 7% of cable faults, cable abrasion for 4%, and component failure (in repeaters) for only 4%.

With the high correlation of faults and maritime activity, it is not surprising that just under one half of cable faults occur in territorial waters, and most of the remainder occur in exclusive economic zones. Just 2% occur on the high seas. Cables are most vulnerable close to shore, in shallow water, where fishing is concentrated, where ships anchor, and where the cable must come ashore at a landing station alongside every other cable serving that coast. The deep ocean, where there is little seabed fishing activity and few ships anchoring, is the safest location for a cable.

National jurisdictions do not stop at the low tide mark of a territory, they extend out into the sea and the seafloor. The 1982 United Nations Convention on the Law of the Sea (PDF) defined sovereign territorial waters as a 12-mile zone extending out from an economy’s coast. Every coastal economy has exclusive rights to the oil, gas, and other resources in the seabed up to 200 nautical miles from shore or to the outer edge of the continental margin, whichever is the further, subject to an overall limit of 350 nautical miles (650km) from the coast or 100 nautical miles (185km) beyond the 2,500-metre isobath (a line connecting equal points of water depth).

What does this mean for submarine cables? From the point where the cable enters territorial waters, any physical activity related to the cable requires the permission of the authorities. All cable construction and repair operations carried out in these sovereign areas also require the permission of the economy that has exclusive rights to the sea and seafloor.

Locating a cable break is straightforward. An optical pulse will be reflected at the point of a cable break, which can provide the distance to the fault from the start of the cable with considerable precision. At that point, it becomes a marine operation.

A specialist cable repair ship must be engaged, which, in turn, must obtain the relevant government’s permission to work at the location of the cable fault (don’t forget that only 2% of outages occur in international waters). Once at the break location, the repair vessel drags a grapnel along the seabed to hook the cable, cuts it, recovers one end, buoys it off, then recovers the other end. A new section is spliced into the cable aboard the ship in a jointing cabin, the fibre fusion-spliced and tested, the joint rebuilt and sealed against pressure, and the repaired cable laid back down with enough slack to reach the seabed.

The cost of a repair can vary greatly, depending on the location and depth of the fault, the nature of the contracted arrangements for cable repair, the amount of time the cable operator is willing to wait for a repair vessel and crew, and the weather. USD 500k to USD 1M is a representative cost of a single cable repair.

While the number of cables has grown, the time taken to obtain permission to operate repairs in territorial waters has generally increased. Industry data indicates repair response times have more than doubled over the past decade, and the longest single repair recorded in 2023 took 947 days, many of which were spent waiting for permission to undertake the repair.

There is also the issue of repair vessels. The industry has roughly 60 ships capable of repairing a submarine cable, operated under maintenance agreements that pool the cost of running this fleet across cable owners in each geographic region. These collective Submarine Cable Maintenance and Repair Agreements are used in the Atlantic Ocean, the North American Western Coastal region, the Mediterranean, South East Asia and the Indian Ocean, a