When a Submarine Cable Breaks, a Ship Fishes It Up and Splices It by Hand

Almost all intercontinental traffic runs through submarine cables, as we covered in our complete guide to submarine cables. That thin line across the ocean is surprisingly fragile — and protected by a remarkable amount of human effort.

How Often Do Submarine Cables Break?

Submarine cable faults are anything but rare. Worldwide, roughly 150-200 faults occur every year (an estimate from the ICPC, the International Cable Protection Committee; TeleGeography puts it at "more than 100 a year").

About two-thirds to 80% of faults are caused by fishing gear and ship anchors. Most happen close to shore, in water shallower than 200m. Natural causes — undersea earthquakes and submarine landslides (turbidity currents) — account for more, while deliberate cuts are rare. With a cable only 17-21mm in diameter in its deep-sea sections lying across vast stretches of ocean, collisions with human activity are impossible to avoid entirely. Cable construction is covered in Where do submarine cables land in Japan? (ICPC / Submarine Networks).

How Is a Broken Cable Repaired?

Repairing a broken cable is a major operation carried out by ships and human hands. Let's follow the steps in order.

First, electrical measurements narrow the fault location down to within a few kilometers. Next, a repair ship sails to the site — the transit alone can take several days — and lowers a grapnel (an anchor-shaped hook) to the seabed to snag the cable. In deep water, the crew first cuts the cable, then raises each end separately with buoys and hauls it on deck. Recovery from a depth of 8,000m takes more than a day.

The recovered ends are then fusion-spliced in the ship's jointing room — the fiber faces are aligned and melted together with the heat of an electric arc. A fresh length of cable is spliced in to make up the slack, the repair is tested, and if all is well the cable goes back to the seabed. The whole process takes roughly 10-20 days. That is how much time and labor it takes to fix a single cable (OneStepPower / Hackaday).

How Many Repair Ships Are There?

Only about 60 ships in the world do this specialized work — and the fleet is aging. Operations are organized under regional maintenance agreements that assign responsibility for each stretch of ocean.

Japan and its surrounding waters fall under the "Yokohama Zone," where vessels such as KDDI Cable Ship's KDDI Cable Infinity stand ready to deploy 24 hours a day, 365 days a year, for up to 50 consecutive days at sea. Who builds, lays, and maintains cables — the difference between manufacturers and cable ships — is covered in detail in Only Four Companies in the World Can Build Submarine Cables (KDDI Cable Ship).

This Is Why Redundancy Matters

Repairs take ten days or more, and repair ships are scarce — which is exactly why the preparation has to happen up front. Operators lay multiple cables along the same corridor, duplicating the route so that if one cable breaks, traffic keeps flowing over another.

This is precisely the same thinking behind terrestrial networks, which are built as redundant meshes that reroute automatically (Why Japan's networks don't go down easily, in Japanese). At sea and on land alike, the assumption is that lines will break — and layer upon layer of preparation keeps the traffic moving anyway (KDDI Cable Ship / KDDI news release).

Key Takeaways

• Submarine cables suffer 150-200 faults a year worldwide. About two-thirds to 80% are caused by fishing gear and anchors, mostly in shallow water.
• Repair follows a set sequence: locate the fault, snag the cable with a grapnel, haul both ends on deck, fusion-splice, test, and relay.
• Recovering a cable from 8,000m depth takes more than a day; a full repair typically takes 10-20 days.
• Only about 60 cable-laying and repair ships exist worldwide, organized under regional maintenance agreements (Japan falls under the Yokohama Zone).
• Because repairs are slow, route duplication with multiple cables (redundancy) is the key to keeping traffic flowing.