When an Undersea Internet Cable Breaks: What Happens Next
One damaged route can be almost invisible to users or a serious regional outage. The difference is what other connections are available.
When an undersea internet cable breaks, that route stops carrying traffic. Network operators try to move data onto other routes while a repair ship is sent to the fault. People may notice nothing, slower international connections, or an outage if there are too few usable alternatives.
The physical repair and the customer experience run on different clocks: a service can recover through another route before the damaged cable is fixed. Conversely, a place heavily dependent on one link can remain disrupted until the cable or another route is restored.
Other routes have enough capacity and traffic moves across them.
Alternative paths are longer or busier, affecting particular destinations.
A region has limited route diversity or several cables are damaged together.
What actually fails?
“Break” is shorthand. A cable may be severed, its glass fibres may be damaged by bending or impact, or an underwater component may fail. Equipment at the landing stations monitors the system and helps locate the fault. The operators then identify which traffic needs another path and organise the marine repair.
Most faults are accidental. The International Cable Protection Committee (ICPC) says around 150–200 submarine telecommunications cable faults occur globally each year, with roughly 70–80% caused by human activity, mainly fishing and ship anchors. Natural hazards and technical failures also occur. A cable outage alone does not establish sabotage.
The damaged cable system or affected section loses its useful transmission path. It does not switch off every network, website or broadband line connected somewhere along its route.
Can the internet simply go around it?
Often, but not automatically. A network may have protected circuits or capacity on geographically separate cables and land routes. It can shift traffic away from the failed path. The quality of that fallback depends on spare capacity, commercial arrangements and whether the alternative cables share the same point of failure.
For a UK household, the local fibre or Wi-Fi connection can still look normal while a distant website or app behaves differently. A longer or congested substitute route may raise latency or reduce performance. There is no universal “all websites slow down” effect: traffic for a nearby service may not use the damaged cable at all.
Places with only one or two international links face a different risk. If a sole route fails or multiple routes are hit, a country or island can suffer a severe outage. That is why route diversity matters more than the headline number of cables. Our guide to why subsea fibre cables matter to the UK internet covers the wider resilience question.
How do crews repair a cable on the seabed?
Repair crews need to locate the damage, reach it by ship, recover the affected cable and join in a sound replacement section. The exact method depends on depth, seabed and whether the fault is in the cable or a repeater.
Landing-station tests narrow down the position and the operator dispatches a suitably equipped repair vessel.
The crew uses a grapnel or remotely operated vehicle where appropriate, brings the ends aboard and removes the damaged section.
Technicians attach spare cable or replace failed equipment, then test the optical and electrical system.
The repaired length is lowered to the seabed; protection or burial may be restored where required.
This is why a fault is not fixed by pressing a reset button. A vessel must be available, reach the site and work in suitable conditions; permissions and logistics can also delay the start. ITU design guidance estimates a mean repair time of one to three weeks for an optical submarine system, while individual repairs can be faster or take longer. That estimate is for physical repair, not a promise about when customers regain service through rerouting.
Traffic restoration may happen as soon as another usable route is available. Physical repair requires marine work and can continue after customers’ connections appear normal.
The takeaway
A broken undersea cable removes one physical route. Other routes may keep services running, sometimes with slower or less reliable connections; areas with little redundancy can lose access. Operators reroute what they can while specialists recover, splice, test and relay the damaged cable.
Undersea cable break FAQs
Does one broken undersea cable shut down the internet?
Usually not. Networks often have other cable routes and can reroute traffic, but an area with few alternatives may suffer a serious outage.
How long does an undersea internet cable take to repair?
ITU design guidance estimates a mean of one to three weeks for physical repair, depending on location, depth, ship availability, cause and weather. Service may return sooner if traffic can use another route.
Can a subsea cable fail without being cut in two?
Yes. Damaged glass fibres, severe bending or a failed underwater component can cause a fault even without a clean physical severing of the cable.
Why might some websites work while others do not?
They can use different destinations and network routes. Traffic affected by a lost cable may take a longer or busier alternative, while other traffic is untouched.
Broadband and telecoms writer