When a communication is sent from a phone in New York to a server in London, most of the time no one considers its physical path. The seamless, instantaneous, and untethered nature of the internet’s infrastructure is part of its architecture. However, a fiber-optic cable the size of a garden hose is working somewhere on the Atlantic Ocean floor, at least a kilometer below the surface, to enable that communication.
Approximately 530 of these cables are currently in use worldwide, and taken as a whole, they transport over 95% of all internet data. These cables are not satellites or radio towers, but rather actual cables that are resting in the silt at the ocean’s bottom and are only held there by gravity. The internet doesn’t go dark when one of them breaks. However, it delays, reroutes, and strains in ways that manifest as outages in locations that were unaware they were reliant on a specific seafloor section off the coast of Virginia or New Jersey.
Three groups comprise the reasons of subsea cable breakage, and the distribution is not what most people anticipate. Between 80% and 90% of cable faults worldwide are caused by human action, not natural disasters or espionage, but rather routine commercial operations that interact with the seafloor in ways that the operators might not even be aware of. The most frequent offender is bottom-trawling fishing gear. In shallower coastal seas where cables haven’t been buried far enough, large nets dragged across the ocean floor by commercial fishing vessels frequently snag wires sitting in the mud.
The second mechanism is anchor dragging, which occurs when cargo ships and container ships drop anchor in places where undersea cable routes cross. When the anchor drags down the bottom before catching, it can cut several cables at once. These are hardly dramatic events. It might not even be noticed by anyone on board the ship. Later, when network engineers notice a flaw in the line, they remotely find the break.
Although it happens less frequently, the geological danger category is dramatic when it does. Underwater canyons are traversed by turbidity currents, which are dense, swift avalanches of mud, sand, and rock that are caused by earthquakes or slope failures. These currents can reach speeds of over 100 km/h and have sufficient force to break steel-jacketed cables. As the avalanche descended the slope following the 1929 Grand Banks earthquake off Nova Scotia, turbidity currents snapped twelve telegraph lines in succession.
Scientists were able to determine the speed of the current by measuring the intervals between each break. That incident is still among the most obvious examples of underwater infrastructure being disrupted by geology. More recently, similar occurrences have been documented along cable corridors off the East Coast of the United States, where sediment builds up in unstable ways and the continental shelf declines sharply.
The third group is the most difficult to resolve clearly and the one that intelligence services and infrastructure security analysts worry about the most. Investigations into whether Russian or Chinese vessels were engaging in what is sometimes referred to as hybrid warfare—deliberately dragging anchors across cable routes while maintaining plausible deniability—have been prompted by a number of cable breaks in European waters in recent years, especially in the Baltic Sea and close to Scandinavia.
The problem is that anchor dragging is also something that ships occasionally do by mistake, and it takes evidence that is rarely conclusive at the bottom of the ocean to discern between accident and purpose. Although it’s still unknown how many of the recorded breakdowns in Western waterways are actually unintentional and how many indicate something more intentional, the trend has been regular enough that governments have started to approach it as a category of danger rather than a string of coincidences.
Redundancy allows the internet to withstand cable outages. Network operators immediately divert traffic over alternate cables or, in certain situations, satellite links when a cable fails. The rerouting is intended to be so quick that the majority of users are unaware of it. Repair ships carry the damaged ends to the surface, attach new fiber-optic strands, and lower the repaired part back to the seafloor. They are specialist vessels with cable-laying and splicing equipment that takes weeks to position perfectly.
At depths where direct human intervention is not feasible, the procedure is costly, sluggish, and demands extreme precision. There is a sense that the physical architecture of the global internet is more exposed than the seamlessness of the service suggests, and that the gap between “covered by redundancy” and “genuinely protected” is wider than the industry tends to acknowledge, as one watches the cable map fill with reported breaks over the past few years.
