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When an undersea internet cable breaks, monitoring equipment at a shore station normally raises an alarm almost immediately. Network operators locate the affected span, notify the appropriate maintenance authority and move traffic onto other cables, landing stations, terrestrial links or satellite capacity where those options exist. The usual result is not a worldwide blackout but reduced capacity, higher latency and congestion on some international routes.
The disruption depends on how many independent paths remain, how much spare capacity they have and where the break occurs. A cut in a well-connected corridor may be barely noticeable; several cuts in a cable chokepoint can create a serious regional outage. Repair then requires a specialized vessel to find, retrieve, splice and test the cable, with travel, permits, weather and vessel availability determining the calendar time.
What happens immediately after a submarine cable breaks?
1. Shore equipment detects an abnormality
Submarine cable systems are supervised from terminal stations on land. Optical and electrical test equipment can detect a sudden loss of signal, a change in transmission characteristics or a power fault. Operators use those measurements to estimate which segment contains the problem and whether the failure is likely to be a complete break, damage to a repeater or a terminal-equipment issue.
2. Operators confirm the affected route
Network control teams compare alarms from both ends of the system and check traffic statistics. They determine which customers, landing points and international links share the failed span. A fault report is sent to the cable owner, consortium, regional maintenance organization and any authority that must coordinate a repair vessel.
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3. Traffic is moved to surviving paths
Carriers change routing and traffic-engineering policies so packets use another cable system, a different landing station, a terrestrial fiber route or, where available, satellite capacity. Automated routing can react in seconds or minutes, while broader capacity planning and customer changes may take longer. The backup route may cross a different ocean basin or add several network hops, so it is often slower and more expensive to operate.
4. Users see degradation rather than silence
People near the affected region may notice slower downloads, higher ping times, buffering or intermittent access to particular destinations. Services whose data is cached locally may continue normally, while applications that depend on a congested international path suffer first. A single cable cut becomes a total loss of connectivity only when no usable alternative reaches the affected location or when remaining paths are overwhelmed.
Why do undersea cables fail?
The International Cable Protection Committee (ICPC) says approximately 150–200 submarine telecommunications cable faults occur worldwide each year, according to its 2025 factsheet. The organization reports that 70–80% result from accidental human activity, especially commercial fishing gear and ship anchors. These activities are most common on continental shelves and in shallow coastal water, where cables are also concentrated near landing stations.
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Fishing and anchors
Trawling equipment can snag or drag a cable, while an anchor can cut, crush or pull it out of its designed position. Burial in the seabed protects many near-shore sections, but storms, seabed movement and repeated contact can expose a previously buried span.
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Geological and seabed causes
Earthquakes, submarine landslides and volcanic activity can break several cables in the same area. Seabed abrasion can wear the protective layers over time, particularly where currents move sediment or rock across the cable. Those events matter because a natural disturbance can affect multiple systems before operators can reroute traffic.
Equipment and installation defects
Repeaters, joints, power-feeding equipment and other components can fail without an external impact. Manufacturing, installation or aging defects are less common than fishing and anchors in the ICPC figures, but they can still require the same vessel-based recovery and splice process.
Will one cable cut take a country offline?
Usually, no. International carriers purchase capacity on multiple cable systems and connect to more than one landing station. The internet is built from interconnected networks, so a failed span can often be bypassed. The ITU describes submarine cables as “the lifelines of the global digital economy, facilitating over 99% of international data exchange” (International Telecommunication Union, 2024 overview). That statistic also explains why operators design multiple paths: most cross-border traffic depends on a relatively small number of physical corridors.
Redundancy is not simply a count of cables. Two systems that land in the same building, follow the same seabed corridor or pass through the same narrow strait may share a common failure point. A provider with geographically separated routes can lose one system and retain more usable capacity than a provider whose “backup” follows the same chokepoint.
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| Resilience factor | What to examine | Why it changes the impact |
|---|---|---|
| Geographic diversity | Whether routes cross different seabed corridors and regions | Separate corridors reduce the chance that one incident cuts every path. |
| Landing-station independence | Number of landing points and whether they share power, backhaul or a site | A second cable is less useful if both systems depend on the same damaged shore facility. |
| Spare capacity | Unused bandwidth available after a fault | Large reserves absorb redirected traffic; small reserves create congestion and packet loss. |
| Backup-path latency | Distance and number of hops on the alternate route | A longer path raises round-trip time even when service remains available. |
| Operational exposure | Permits, protected waters, repair-vessel access and political constraints | These factors determine how quickly a damaged system can be restored. |
When several cables share a chokepoint
A narrow channel, landing area or heavily used seabed corridor can contain many cables. An anchor incident, construction project or geological event there can damage more than one system. Providers may retain connectivity through distant routes, but the lost capacity is harder to replace and latency rises sharply. This is why route maps that show only national endpoints can overstate practical resilience.
How is internet traffic rerouted?
Carrier routing and traffic engineering
Each network maintains routing policies that select available paths. When a cable session or transport link disappears, routing protocols withdraw the failed path and advertise alternatives. Operators can also reserve capacity, rate-limit lower-priority traffic or steer selected destinations through a different backbone. These controls preserve essential connectivity but cannot create bandwidth that does not exist.
Alternate landing stations and terrestrial links
A cable system may connect to several coastal stations. Traffic can be carried over inland fiber between a surviving station and the network core, or across a neighboring country whose international gateway remains healthy. Such detours depend on interconnection agreements, compatible equipment, available power and physical capacity on the terrestrial route.
Satellite as a limited substitute
Satellite links can provide emergency or hard-to-reach connectivity when terrestrial and submarine alternatives are unavailable. They generally offer far less aggregate capacity than a major fiber cable and may have different latency, spectrum and weather constraints. Operators therefore treat satellite as a supplement rather than a like-for-like replacement for an oceanic fiber system.
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Why a reroute can feel slow
Suppose a direct cable between two regions fails and traffic is sent around another continent. The surviving path has a longer physical distance and may already carry normal traffic. Round-trip times increase, queues form at busy links and some providers may temporarily reduce throughput. The Red Sea cable cuts described by the Associated Press illustrate this pattern: connectivity continued, but providers lost part of their international transit and users faced lower available capacity and higher latency.
How a damaged cable is repaired
Repair vessels work through regional maintenance agreements. The vessel, crew and spare cable are typically arranged in advance, but a ship may be serving another incident when a new fault occurs. A normal repair follows these stages:
- Locate the fault from shore. Test equipment at the terminal stations measures the cable electrically and optically to estimate the distance to the defect. Measurements from both ends narrow the search area.
- Obtain clearances and mobilize a vessel. The owner or maintenance authority assigns a suitable ship, checks its grappling and jointing equipment and secures permissions for the route, fishing areas, territorial waters and protected zones.
- Sail to the estimated position. Travel time depends on the vessel’s port, weather, sea state and the distance to the fault. A ship may leave quickly yet still need days to reach an oceanic site.
- Grapple and recover the cable. The vessel lowers a grapnel to catch the cable on the seabed and hauls it to the deck. Deep-water work can require repeated attempts, and rough conditions may force a pause.
- Cut out the damaged section and splice in spare cable. Technicians remove the failed length, bring the two healthy ends aboard and add spare cable. They make a fiber joint in a controlled workspace; ITU-T design guidance says systems should be repairable and that onboard jointing should be reasonably short in good conditions.
- Test, lower and return to service. The repaired span is tested for optical loss, power and signal performance. Once readings meet the system’s requirements, the crew lowers the cable back to the seabed, shore teams restore normal routing and operators monitor the link for stability.
How long does a submarine-cable repair take?
“Repair time” has several meanings. The clock may start when the fault is reported, when a vessel leaves port, when it reaches the site, when the physical splice is completed or when traffic is fully restored. Those milestones can be separated by days or weeks, so a promised repair date is meaningful only when it names the milestone.
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| Situation or milestone | Published timing | What the figure means |
|---|---|---|
| Vessel mobilization | Within 24 hours of notification | UK parliamentary evidence says repair ships would leave port within 24 hours when notified; this is departure, not arrival or restoration. |
| Irish Sea example | Up to two days’ travel plus five days’ repair | A nearby fault could therefore require about a week under the cited example’s conditions. |
| Mid-Atlantic example | Seven days’ travel plus ten days’ repair | An oceanic incident can take roughly 17 days before counting additional permissions, testing or weather delays. |
| Bad-weather operation | About a month | UK evidence notes that poor conditions can extend the overall operation substantially. |
| Longest 2023 repair in the cited ICPC analysis | 947 days | This exceptional outlier, reported by Submarine Networks from ICPC Global Cable Repair Data Analysis, shows that some records include prolonged access, coordination or restoration problems. |
The ITU recorded more than 200 cable repairs in 2023, or more than three per week, in its 2024 overview. An ICPC analysis reported by Submarine Networks found an average of 199 faults per year from 2010–2024 and 206 repairs in 2023. These totals describe industry-wide activity, not a standard duration for an individual incident.
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What causes the longest delays?
- Distance and vessel availability: A ship may be stationed far from the fault or committed to another repair.
- Permits and coordination: Territorial waters, environmental rules, military zones, fishing activity and multiple cable owners can delay authorization.
- Weather and sea state: Grappling and deck jointing may be unsafe in heavy seas, forcing the vessel to wait or shelter.
- Difficult seabed conditions: A cable buried under sediment, entangled with debris or damaged over a wide area may require repeated grappling attempts.
- Multiple simultaneous faults: Several breaks compete for the same ships, spares and specialist crews.
- Restoration after the splice: A physical joint can be complete while testing, power checks, route changes and customer reconnection continue.
What should internet users expect during an outage?
Expect uneven effects rather than a single “internet on” or “internet off” condition. Local websites, cellular services and data centers with nearby caches may work normally, while international video, cloud applications, gaming servers or business links on the congested route become slow. A provider may impose temporary traffic limits or prioritize critical services until additional capacity is arranged.
For a household, changing a Wi-Fi setting cannot repair a submarine fault. The practical steps are to check the provider’s status page, use an available alternate connection for urgent work and avoid repeatedly restarting equipment when the problem affects multiple destinations. Organizations that need continuity should contract diverse carriers and landing stations, verify that their backup paths do not share a corridor and test failover before an incident.
The practical answer
A broken undersea cable triggers detection, diagnosis and rerouting first; specialized marine work follows. Most single failures reduce capacity or increase latency because surviving paths carry the traffic, but they do not disconnect an entire country. Fishing and anchors cause most faults, especially in shallow water. A nearby repair may take about a week, an oceanic repair several weeks, and exceptional cases far longer. The decisive protections are geographically independent routes, truly separate landing points and enough spare capacity to carry traffic while a vessel restores the failed span.
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