The global internet feels wireless, but most data travelling between countries crosses the ocean through fibre-optic cables. These lines carry video calls, cloud traffic, financial messages and ordinary web requests between landing stations on different coasts.
Satellites are important for remote coverage and backup, but cables provide enormous capacity with low delay. The system remains reliable because it is a network of many routes rather than one unbroken line.
Light carries data through glass
Inside a submarine cable, optical fibres transmit information as pulses of light. Protective layers surround the fibres, with heavier armour used near shore where anchors and fishing activity create greater risk.
Across long ocean spans, repeaters placed along the route amplify the signal. Power supplied from landing stations keeps that equipment operating thousands of metres below the surface.
Landing stations connect oceans to cities
Cables come ashore at secure landing sites and connect to domestic fibre networks and data centres. A country's resilience depends not only on the number of cables but also on whether they use different landing points and geographic paths.
Several cables entering the same beach or crossing the same narrow corridor can share one vulnerability. Diverse routes reduce the chance that a single event isolates the network.
Why cables break
Most faults occur in relatively shallow water. Fishing gear and ship anchors can drag across the seabed, while earthquakes, underwater landslides and abrasion also cause damage. Equipment can fail without a dramatic external event.
A break does not necessarily disconnect users. Network operators reroute traffic through other cables, although congestion can slow services or increase latency. Islands and countries with few international links face greater risk.
How engineers locate a fault
Operators test the cable by sending light pulses and measuring reflections. The timing helps estimate the distance to the damaged section. Maintenance agreements then dispatch a specialised repair ship carrying spare cable and equipment.
Weather, permits and the distance to port affect response time. Repair ships may also have to avoid other infrastructure and coordinate with maritime authorities.
How a cable is repaired
In shallower water, a grapnel can recover the cable from the seabed. Deep-water work may use remotely operated vehicles. The damaged section is cut out, new fibre is spliced in and the link is tested before being lowered back.
The process is precise: hair-thin fibres must be aligned while the cable remains strong enough for deployment. A repair can take days or longer when conditions are difficult.
Who owns the network?
Telecommunications groups, infrastructure companies and large technology firms invest in cable systems, often through consortia. Capacity is divided or sold to carriers and cloud providers.
Routes also have strategic importance. Governments monitor landing sites, security and repair capability because communications infrastructure supports finance, defence and public services.
The bottom line
Undersea cables keep countries online by moving vast amounts of data through optical fibre. Redundant routes allow traffic to bypass many faults, while specialised ships repair physical damage. True resilience requires multiple cables, diverse landing points and strong domestic connections. Read more global infrastructure coverage in World and our explainer on satellite-to-phone messaging.
