Earlier this week, operators of two communications cables—C-Lion 1 and BCS East-West Interlink—reported faults in the Baltic Sea.
Local internet service seems largely unaffected, but a swirl of sabotage allegations have emerged in the global press.
Without making a ruling on any ongoing investigations, let’s look at the facts.
How unusual are cable breaks?
Submarine cables break all the time. On average, two to four break somewhere in the world every week.
While damage is more common in some areas than others, these breaks—or “faults”—eventually happen to almost every cable.
If cables go out of service so often, why don’t I hear about it?
Most of the time, cable faults only make the news if multiple systems go offline or damage occurs in geopolitical hotspots. Simultaneous breaks are more likely to affect service quality, and some islands may lose their undersea connectivity altogether (like in the Shetland and Matsu Islands).
You rarely hear about other cable faults because most telecom providers follow a “safety in numbers” approach.
By spreading their networks’ capacity over multiple cables, operators ensure that if one breaks, their network will run smoothly over the others until the damage is repaired.
By spreading their networks’ capacity over multiple cables, operators ensure that if one breaks, their network will run smoothly over the others until the damage is repaired. This is called network redundancy.
To minimize downtime, cable owners also hold standing agreements with maintenance providers that keep ships on standby, waiting to deploy spares of extra cable length and make needed repairs quickly.
What causes cables to break?
Most faults are caused by “external aggression.”
That sounds scary! But this term only means that a cable didn’t suffer a technical fault on its own, and was instead damaged by external forces. Most come from fishing equipment, normal anchoring activity, and natural disasters like undersea earthquakes. Internal component or equipment failure causes another, smaller category of faults.
(In case you’re wondering, no cable faults have been attributed to shark bites since 2007.)
Although multiple cables breaking at the same time feels improbable, it’s more likely than you may think.
In some locations, geological or regulatory constraints herd cables into narrow corridors, greatly increasing the risk of simultaneous cuts across multiple cables. Across larger distances, multiple breaks can happen through simple bad luck.
Are cables ever damaged on purpose?
State-sponsored sabotage is extremely rare, and most publicly known examples are decades old (for example, in World War I or the Spanish-American War).
However, routine cable faults can resemble sabotage if operators or governments aren’t certain of what caused them. After all, deniability is a key element of modern “gray zone” attack vectors.
While less exciting, some fishermen may cut a cable on purpose. This can happen if their gear snags on a cable and they are able to bring it to the surface. To discourage this, cable operators often offer to pay for lost equipment if fishermen sacrifice their gear instead of tampering with the cable.
How do we know what happened to a cable?
In cases like Tonga’s, where a volcanic explosion famously disrupted the island nation’s only subsea connectivity, it’s easy to conclude that environmental damage caused a break.
In other instances, determining the cause of a break takes time. Maintenance crews often set sail knowing where damage occurred—but not what caused it. Once on-site, initial hypotheses can be confirmed or denied based on how the cable looks.
One tool cable operators can use to determine the possible cause of damage from afar is Automatic Identification System (AIS) data.
Ships use AIS to transmit their location back to shore or to other nearby vessels. This helps ensure safe navigation and is required for some ships under international law. If a ship crosses a cable at the same place and time that it breaks, that ship may have caused the damage (accidentally or otherwise).
So if AIS shows a foreign ship crossing a cable, that’s evidence of sabotage?
When AIS shows a vessel passing back and forth over a cable right before it breaks, it can look suspicious.
Sometimes, this is just mundane fishing activity—e.g., a trawler making multiple passes over the seabed to scoop up fish. It may even be legal, as not all countries have protected zones around cables.
AIS data can also indicate where a vessel is flagged. However, it’s not uncommon for fishermen to find their catch well beyond their home country’s shores. China, for example, has the world’s largest fishing fleet, with vessels routinely traveling across the globe.
AIS data isn’t always available. Sometimes, AIS is turned off on purpose in order to illegally fish in certain areas. Other times, transmitted data might not be picked up by receivers. (AIS is limited in range because it‘s transmitted wirelessly.)
What’s next?
Official determination of a fault’s cause is best left to operators, maintenance crews, and government investigators.
There are steps governments and cable developers can take to help reduce faults, including liaising with fishermen, burying cables near shorelines, and promoting cable diversity. To learn more, check out the ICPC’s list of best practices.
Meanwhile, we can find comfort in remembering that extraordinary circumstances and coincidences happen every day, and that most cable faults are just bad luck.
For more data on cable damage, subscribe to TeleGeography’s Transport Network Research Service, which features our database of cable faults.
