There are 1.4 million kilometres of telecommunications cables on the seabed, spread across all the planet’s oceans.
99% of the world’s digital communications rely on undersea cables. If they break, they could end up causing a disaster for the internet of an entire country. How do you fix a fault at the bottom of the ocean?
There are 1.4 million kilometres of telecommunications cables on the seafloor, spread across all of the planet’s oceans. If stretched out in a straight line, these cables would be long enough to span the diameter of the Sun. For something so large, they are surprisingly thin – often just over 2 cm in diameter, or about the width of a hose.
The repair and maintenance of this cabling system – which began to be implemented in the mid-19th century and continues to be expanded and updated to this day – has led to other types of astonishing scientific discoveries, opening up entirely new worlds and allowing us to spy on the seabed like never before, as well as communicate at record speed.
At the same time, our daily lives, our income, our health and our security have also become increasingly dependent on the internet and ultimately on this complex network of undersea cables. So what happens when they break?
“For the most part, the global network is remarkably resilient,” says Mike Clare, a marine environmental adviser at the International Cable Protection Committee who researches the impacts of extreme events on subsea systems.
“There are between 150 and 200 cases of damage to the global grid every year. So if you compare that to 1.4 million kilometers, that’s not a lot, and for the most part, when this damage occurs, it can be repaired relatively quickly.”
But how does the internet work through such thin cables, and how can disastrous outages be avoided?
Since the first transatlantic cables were laid in the 19th century (using the telegraph era), cables have been exposed to extreme environmental events, from underwater volcanic eruptions to typhoons and floods. But most of the causes of damage to cables are not natural.
Most failures, with figures varying between 70% and 80% depending on where in the world they occur, are related to accidental human activities such as dropping anchor or dragging nets from fishing boats, which end up caught in the cables, says Stephen Holden, head of maintenance for Europe, the Middle East and Africa at Global Marine, a subsea engineering company that deals with subsea cable repairs.
Typically, these accidents occur at depths of 200 to 300 m (although commercial fishing is moving into deeper waters, in some places reaching 1,500 m in the northeast Atlantic).
Natural hazards account for only 10% to 20% of cable failures worldwide, and most often they involve cables fraying in places where currents cause them to rub against rocks, causing what are called “shunt failures,” Holden says.
(The idea that cables break because sharks bite them is now something of an urban legend, Clare adds. “There have been cases of damage caused by shark bites, but that has since passed because the cable industry uses a coating to strengthen cables.”)
However, cables must be kept thin and light in deeper waters to aid recovery and repair. Transporting a large, heavy cable thousands of metres below sea level would put enormous strain on it. Cables closer to shore tend to be better armoured because they are most likely to get caught in nets and anchors.
If a fault is found, a repair ship is sent. “All of these vessels are strategically placed around the world to be within 10-12 days of their base,” says Mick McGovern, deputy vice president of maritime operations at Alcatel Submarine Networks.
“That gives you the time to determine where the fault is, load the cables [and] repeaters,” which boost the strength of a signal as it travels along the cables. “In essence, when you think about the size of the system, there’s not much to wait for ,” he says.
McGovern says a modern deepwater repair should take a week or two, depending on location and weather. “When you think about the depth of the water and where it is, it’s not a bad fix.”
That doesn’t mean an entire country will be without internet service for a week. Many nations have spare cables and bandwidth that exceeds the minimum amount required, so if some are damaged, the others can take over. This is called system redundancy. Because of this redundancy, most of us would never notice if an undersea cable was damaged; this article would probably take a second or two longer than usual to load.
In fact, in extreme events, these cables can be the only thing keeping a country online. The 2006 magnitude 7 earthquake off the coast of Taiwan severed dozens of cables in the South China Sea, but a handful of them remained online.
To repair the damage, the ship deploys a grappling hook to lift and cut the cable, pulling a loose end to the surface and reeling it in along the bow using large motorized drums. The damaged section is then dragged into an internal room and analyzed for faults, repaired, tested by sending a signal ashore from the ship, sealed, and then secured to a buoy while the process is repeated at the other end of the cable.
Once both ends are attached, each optical fiber is spliced under a microscope to ensure a good connection, and then sealed with a universal joint that is compatible with any manufacturer’s cable, making life easier for international repair crews, McGovern says.
Repaired cables are returned to the water and, in shallower waters where there might be more ship traffic, they are buried in trenches. Remotely Operated Underwater Vehicles (ROVs), equipped with high-powered jets, can cut paths in the seabed to lay cables. In deeper waters, the work is done by ploughs equipped with jets that are dragged along the seabed by large repair ships. Some ploughs weigh more than 50 tonnes and even these are too small for more extreme work.
McGovern recalls a job in the Arctic Ocean that required a ship to pull a 110-tonne plough, capable of burying cables 4 metres deep and penetrating permafrost.
The laying and repair of cables has led to some surprising scientific discoveries – at first somewhat accidentally, as in the case of underwater landslides.
In 1929, a 7.2 magnitude earthquake struck off the coast of Canada’s Burin Peninsula. The powerful quake triggered a tsunami that killed 28 people. It also severed what was then a revolutionary communications technology: at least 12 transatlantic submarine cables were ruptured in 28 different locations.
By analyzing the broken cables, it was possible to identify that some of the breaks occurred at the time of the earthquake, while about 16 occurred over an extended period of time, and in a wave pattern.
If all the cables had been broken by the earthquake, they would have broken at the same time, so scientists began to wonder why there were such particular breaks.
It was not until 1952 that researchers discovered why the cables had snapped in sequence, over such a large area and at intervals that appeared to decrease with distance from the epicenter.
They discovered that a landslide had passed through them: its movement had been traced by cables breaking on the seabed. Until then, no one knew of the existence of turbidity currents: landslides on the seafloor caused by the accumulation of sediment in the water due to events such as an earthquake, causing it to flow downward, like snow in an avalanche.
The deployment of the cables has also led to discoveries made intentionally, when scientists began using the cables as research tools.
These lessons from the deep sea began when the first transatlantic cables were laid in the 19th century. Cable operators noticed that the Atlantic Ocean became shallower in the middle, inadvertently uncovering the Mid-Atlantic Ridge.
Today, telecommunications cables can be used as “acoustic sensors” to detect whales, ships, storms and earthquakes on the high seas.
The damage to the cables gives the industry “a fundamental new understanding of the dangers that exist in the deep sea,” Clare says. “We would never have known that there were landslides under the sea after volcanic eruptions if it wasn’t for the damage that occurred [to the cables].”
In some places, climate change is making things more challenging. Flooding in West Africa is leading to increased canyon outflow in the Congo River, which occurs when large volumes of sediment flow into a river after a flood. This sediment then washes out of the river mouth into the Atlantic and could damage cables. “We now know that we need to lay cables farther away from the estuary,” McGovern says.
Some damage will be unavoidable, experts predict. The Hunga Tonga-Hunga Ha’apai volcanic eruption in 2021-2022 knocked out the undersea internet cable linking the island nation of Tonga to the rest of the world. It took five weeks to fully repair its internet connection, though some services were restored after a week.
However, many countries have multiple undersea cables, meaning internet users might not notice a fault, or even multiple faults, as the network can fall back to other cables in the event of a crisis.
“This really shows why geographic diversity of cable routes is necessary ,” Clare adds. “Particularly small islands in places like the South Pacific, which have tropical storms, earthquakes and volcanoes, are vulnerable, and with climate change, different areas are affected in different ways.”
As fishing and shipping become more sophisticated, it may become easier to avoid cables. The advent of automatic identification systems (AIS) in shipping has made it possible to reduce damage caused by anchoring, Holden says, because some companies now offer a service where a set pattern can be followed to slow down and anchor.
But in parts of the world where fishing vessels tend to be less sophisticated and operated by smaller crews, anchor damage still occurs.
In such places, one option is to inform people about where the cables are and raise awareness, Clare adds: “It is in everyone’s interest that the internet continues to work.”
