How Subsea Internet Cables Work
From a landing station to the seabed and back: the physical route behind an international internet connection.
A subsea internet cable is a fibre-optic link laid across the seabed between coastal landing points. Equipment on land sends data as light through hair-thin glass fibres. On long routes, powered devices along the cable amplify that light so it can reach the other shore.
It is part of a much larger network, not a cable running from your house straight into the sea. Your ISP and other networks decide which path a connection takes. A UK-to-UK service may never cross an ocean, while a service hosted abroad may use a subsea route.
The return data travels through a network path too; it does not necessarily follow the exact same route in reverse.
What is actually under the water?
The data travels through optical fibres in the centre. Around them are materials that protect the fibres, give the cable strength and, on powered systems, carry electricity to subsea equipment. The construction changes with the route: a shore section exposed to anchors and fishing gear needs different protection from a deep-ocean section.
- Outer coveringInsulation and protection from seawater and abrasion.
- Armour or strengthSteel reinforcement where the route needs it, especially near shore.
- Power pathA metallic conductor can feed electricity to repeaters on long systems.
- Glass fibresThin optical strands carry the light signals containing data.
Different light wavelengths can carry multiple channels through the same fibre. This is one reason a cable with a small physical diameter can transport huge amounts of traffic. The capacity depends on the number of fibre pairs, the optical equipment and the system design; there is no single speed for every subsea cable.
How does the data cross an ocean?
At a landing station, terminal equipment converts the network’s data into optical signals and launches them into the fibres. The fibres guide light along the route. Because signal power falls over distance, transoceanic systems use repeaters at intervals. Modern repeaters are optical amplifiers: they boost the incoming light without decoding every web page or turning each signal back into electronic data.
The repeaters need electricity, which is supplied from shore through the cable’s conductor. At the far landing station, receiving equipment passes the data into terrestrial networks. Some shorter submarine links can work without repeaters; their amplification is provided at or from the ends rather than by powered amplifiers spaced along the seabed.
The optical fibres carry data. The electrical feed powers submerged amplifiers. These are separate jobs within the same cable system.
How is a cable laid and what happens if it breaks?
Operators survey a route and plan its shore landings, seabed conditions and crossings. A specialist cable ship then pays the cable out along that route. In vulnerable shallow areas it may be buried beneath the seabed; on suitable deep-ocean sections it can lie on the seabed. Protection varies with local risk.
Survey the route, select cable construction and install it from a cable ship.
Operators watch system performance and locate a fault when a cable or component fails.
A repair ship retrieves the affected section, replaces or repairs it, tests the joint and lays it back.
Fishing activity and dragged anchors are major causes of faults, particularly in shallower waters. Networks can often route traffic over other cables while a fault is repaired, but the effect depends on how many alternative routes and how much spare capacity are available. For the UK resilience story, read why subsea fibre cables matter to the UK internet.
Subsea cables are the transport layer, not the whole internet. A separate guide to dark fibre explains what it means for fibre to exist without active equipment lighting it.
The takeaway
A subsea cable links networks on opposite shores with glass fibres that carry data as light. Landing stations provide the optical equipment, and long routes use electrically powered repeaters to keep the signal strong. The cable is one route within a wider internet network, so whether your traffic crosses it depends on its destination and how networks route it.
Subsea internet cable FAQs
Do subsea internet cables carry data or electricity?
Both can be present in one system. Glass fibres carry data as light, while a metallic conductor supplies power to underwater repeaters on long routes.
Why do subsea cables need repeaters?
Light loses strength as it travels through fibre. Long cables use powered optical amplifiers called repeaters to boost the signal at intervals; some shorter links do not need subsea repeaters.
Are subsea internet cables buried?
Some sections are buried, particularly near shore where human activity poses greater risk. Cable can also lie on the seabed where the route and conditions allow.
Does every internet connection use a subsea cable?
No. A connection to a nearby service can stay on land. International traffic crossing an ocean commonly uses subsea fibre, but its actual path depends on the networks involved.
Broadband and telecoms writer