I saw several articles voicing concern about sabotage when two different undersea fiber operators, C-Lion and BCS East-West Interlink, reported breaks in fiber in the Baltic Sea in the same week. There was speculation that Russia was cutting fibers to try to disrupt European broadband. It was eventually reported that the cuts looked like accidents, but conspiracy theorists still like the sabotage story better. Having two cables broken in the Baltic Sea got headlines because of tensions caused by the war in Ukraine. To put the Baltic Sea fiber cuts into perspective, there are two to four cable cuts to undersea fiber somewhere in the world every week.

Interestingly, a fiber cut to an undersea fiber doesn’t cause as much harm as most people imagine. This map that shows all of the current submarine cable routes. There are a huge amount of redundant routes to most of the world. A single fiber getting cut is an inconvenience and not a huge problem. Even if all of the fibers in the Baltic Sea were cut, Internet traffic would still be delivered through long-haul fiber routes across Europe.

There are exceptions, and there are island nations that can be isolated by even a single fiber cut. Multiple fiber cuts can cause localized slowdowns. There were four cable cuts off Africa in a relatively short time in 2024 that caused broadband outages in Ivory Coast, Liberia, and Benin. It’s more of a challenge in Africa to reroute traffic using landline fiber since much of the continent still has inadequate middle-mile and long-haul fiber routes.

There are a wide variety of ways that undersea cables get cut. The two most predominant causes are fishing vessels and fibers snagged by anchors. Breaks can come from natural causes like earthquakes, volcanos, or heavy seas. Fibers are cut more often in relatively shallow water than in the deep water in the middle of oceans.

The constantly weekly cuts to fiber have spawned a naval repair industry of ships that constantly circle the globe to fix cable breaks. While that might sound like an exotic job, fixing fibers in the Baltic Sea in February sounds like it deserves hazard pay to me.

The process of repairing cut fibers is interesting. In shallow water, the repair ships locate and grab the end of the cut fibers using ROVs (remote operated vehicles). The mini-submarines grab the fiber with robotic claws and drag the fiber to the surface.

Repairing fibers in deep water is harder. The repair ships locate the fiber using sonar and voltage drop test equipment to locate the ends of the cut fiber. Anybody who locates buried fiber would be intrigued by the process. They then use grapnels, which are large hooks, to snag the fiber and pull it to the surface.

Once the two fiber ends are retrieved, the repair process would be familiar to any fiber field technician. The one difference is that long-haul fiber routes have periodic light repeaters built into the fiber, so it’s more challenging if one of those in part of the repair.