Quick Read: How our data travels beneath the sea...

There are 1.4 million km (870,000 miles) of telecommunication cables on the seafloor, covering every ocean on the planet. Laid end to end, these cables would span the diameter of the Sun, and are responsible for the transfer of 99% of all digital data. But for something so important, they are surprisingly slender – often little more than 2cm in diameter, or about the width of a hosepipe.

Submarine cables form a global web at the bottom of the sea, keeping us all connected (Credit: Getty)

"There are 150 to 200 instances of damage to the global network each year. So if we look at that against 1.4 million km, that's not very many, and for the most part, when this damage happens, it can be repaired relatively quickly."says Mike Clare, the International Cable Protection Committee's marine environmental advisor.

How are disasterous outages avoided?

Since the first cables were laid in the 19th Century, they have been exposed to extreme environmental events, from submarine volcanic eruption to typhoons and floods. But the biggest cause of damage is not natural. The idea that cables break because sharks bite through them is now a bit of an urban legend. Most faults, with figures varying 70-80% depending on where you are in the world, relate to accidental human activities like the dropping of anchors or dragging of trawler boat nets, which snag on the cables, says Stephen Holden, head of maintenance for Europe, the Middle East and Africa at Global Marine, a subsea engineering firm who respond to subsea cable repairs. These usually occur in depths of 200-300m (but commercial fishing is increasingly pushing into deeper waters – in some places, 1,500m in the Northeast Atlantic). Only 10-20% of faults worldwide relate to natural hazards, and more frequently relate to cables wearing thin in places where currents cause them to rub against rocks, causing what are called "shunt faults", says Holden.

Cables have to be kept thin and light in deeper waters, though, to aid with recovery and repair. Hauling a large, heavy cable up from thousands of metres below sea level would put a huge amount of strain on it. It's the cables nearer the shoreline that tend to be better armoured because they are more likely to be snagged by nets and anchors.

An army of stand-by repair ships 

If a fault is found, a repair ship is dispatched. "All these vessels are strategically placed around the world to be 10-12 days from base to port," says Mick McGovern, deputy vice-president for marine operations at Alcatel Submarine Networks. "You have that time to work out where the fault is, load the cables [and the] repeater bodies" – which increase the strength of a signal as it travels along the cables. "In essence when you think how big the system is, it's not long to wait," he says.

While it took nine months to repair the last of the subsea cable damage caused by the 1929 Newfoundland earthquake, McGovern says a modern deep-water repair should take a week or two depending on the location and the weather. "When you think about the water depth and where it is, that's not a bad solution."

That does not mean an entire country's internet is then down for a week. Many nations have more cables and more bandwidth within those cables than the minimum required amount, so that if some are damaged, the others can pick up the slack. This is called redundancy in the system. Because of this redundancy, most of us would never notice if one subsea cable was damaged – perhaps this article would take a second or two longer to load than normal. In extreme events, it 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 remained online.

In deep waters, giant underwater ploughs dig trenches for the cables

To repair the damage, the ship deploys a grapnel, or grappling hook, to lift and snip the cable, pulling one loose end up to the surface and reeling it in across the bow with large, motorised drums. The damaged section is then winched into an internal room and analysed for a fault, repaired, tested by sending a signal back to land from the boat, sealed and then attached to a buoy while the process is repeated on the other end of the cable.

Once both ends are fixed, each optical fibre is spliced together under microscope to make sure that there is good connection, and then they are sealed together with a universal joint that is compatible with any manufacturer's cable, making life easier for international repair teams, McGovern says. The repaired cables are lowered back into the water, and in shallower waters where there might be more boat traffic, they are buried in trenches. Remotely operated underwater vehicles (ROVs), equipped with high-powered jets, can blast tracks into the seabed for cables to be laid into. In deeper waters, the job is done by ploughs which are equipped with jets and dragged along the seabed by large repair vessels above. Some ploughs weigh more than 50 tonnes, and in extreme environments, bigger equipment is needed – such as one job McGovern recalls in the Arctic Ocean which required a ship dragging a 110-tonne plough, capable of burying cables 4m and penetrating the permafrost. 

Ears on the sea floor

Laying and repairing the cables has led to some surprising scientific insights – at first somewhat accidentally, as in the case of the snapped cables and the landslide, and later, by design, as scientists began to intentionally use the cables as research tools.

These lessons from the deep sea began as the first transatlantic cables were laid in the 19th Century. Cable layers noticed that the Atlantic Ocean gets shallower in the middle, inadvertently discovering the Mid-Atlantic Ridge. Today, telecommunication cables can be used as "acoustic sensors" to detect whales, ships, storms and earthquakes in the high seas.

The damage caused to cables offers the industry "fundamental new understandings about hazards that exist in the deep sea," says Clare. "We'd never have known that there were landslides under the sea after volcanic eruptions if it wasn't from the damage that was created."

In some places, climate change is making matters more challenging. Floods in West Africa are causing an increase in canyon-flushing in the Congo River, which is when large volumes of sediment flows into a river after flooding. This sediment is then dumped out of the river mouth into the Atlantic and could damage cables. "We know now to lay the cable further away from the estuary," says McGovern.

Credit: https://www.bbc.com/future/article/20241014-the-deep-sea-emergency-service-that-keeps-the-internet-running