Submarine cables are fiber optic lines that run along the ocean floor, carrying over 95% of all international data and voice traffic between continents. Every time you send an email overseas, stream a video hosted on another continent, or make an international call, that data almost certainly travels through one of these cables. As of 2024, more than 500 active and planned submarine cable systems span the globe, and roughly 200,000 kilometers of new cable were installed in that year alone.
What’s Inside a Submarine Cable
A modern submarine cable is surprisingly thin, roughly the diameter of a garden hose in deep water sections. At its core are strands of optical fiber, each capable of transmitting enormous volumes of data as pulses of light. These fibers are wrapped in layers of protection: a copper or aluminum tube that carries electrical power, insulation, steel wire armor for strength, and an outer polyethylene sheath. In shallow water near coastlines, cables get additional armoring to protect against anchors, fishing gear, and other hazards, which makes them thicker and heavier.
The number of fiber pairs inside a single cable varies by design. Newer, high-capacity systems pack more fiber pairs into a single cable to increase total throughput. AWS’s Fastnet cable, a transatlantic line connecting the United States and Ireland, delivers over 320 terabits per second of capacity. That’s enough to stream roughly 12.5 million HD films at the same time through a single cable system.
How Signals Travel Thousands of Miles
Light signals weaken as they travel through fiber. After roughly 50 to 150 kilometers, the signal has lost enough strength that it needs a boost. That’s the job of repeaters, devices spaced along the cable that amplify the optical signal and send it on its way. A cable stretching 6,900 kilometers might contain dozens of these repeaters, each compensating for the signal loss in its segment. For shorter spans of around 50 kilometers between repeaters, each amplifier provides about 8.5 decibels of gain. Longer spans of 150 kilometers require amplifiers capable of around 25 decibels of gain to make up for the greater signal loss.
These repeaters need electricity, but there are no power outlets on the ocean floor. Instead, high-voltage direct current is fed into the cable from shore stations at each end. The electrical power travels through a copper conductor that runs alongside the optical fibers. Voltages typically reach 15,000 volts, with some systems pushing up to 18,000 volts. This power is shared among all the repeaters along the entire cable length, which makes electrical power one of the most constrained resources in cable design. Adding more fiber pairs to a cable means splitting that fixed power budget among more amplifiers, so engineers constantly balance data capacity against available electricity.
How Cables Are Laid and Protected
Specialized cable-laying ships carry thousands of kilometers of cable on massive spools and pay it out slowly onto the seafloor. In deep ocean areas, the cable simply rests on the seabed. In shallower coastal zones where human activity poses a risk, cables are buried in trenches one to three meters below the seafloor using remotely operated plows or water jets. The installation process is the most disruptive phase of a cable’s life, involving increased vessel traffic, excavation of the seabed, and temporary disturbance to the surrounding environment.
Once in place, submarine communications cables are relatively benign. They produce no chemical emissions. Unlike submarine power cables, which carry large amounts of electricity between land masses, communications cables generate negligible heat and electromagnetic fields. Studies evaluating these effects, including assessments by NOAA and the OSPAR Commission (which monitors the northeast Atlantic), have not found demonstrably adverse impacts on marine organisms from operational communications cables. The main environmental concerns remain limited to the installation period: temporary habitat disturbance, sediment disruption, and the localized physical change to the seabed where cables are buried.
What Happens When a Cable Breaks
Cable faults happen regularly, most often caused by fishing trawlers and ship anchors in shallow water. Earthquakes, underwater landslides, and even shark bites account for a smaller share. When a break occurs, engineers use a technique called time domain reflectometry (TDR) to pinpoint the fault’s location. This works by sending an electrical pulse down the cable and measuring how long it takes for the reflection to bounce back, similar to sonar. The return signal reveals the distance to the break, sometimes with remarkable precision.
Once the fault is located, a specialized cable repair ship travels to the site. In deep water, the ship uses a grapnel to hook the cable on the ocean floor, haul it to the surface, cut out the damaged section, and splice in a new segment. The whole process can take weeks, depending on weather, water depth, the ship’s travel time to the site, and permitting requirements. In U.S. waters, for instance, environmental permits can add weeks or months: consultations with fisheries agencies for endangered species impacts alone can take three to six months, and certain marine wildlife authorizations may require nine to twelve months of lead time. That’s one reason cable operators often pre-position repair ships and maintain stockpiles of spare cable in strategic locations around the world.
Most major cable routes have built-in redundancy. If one cable fails, internet traffic automatically reroutes through other cables on the same or nearby paths. Users rarely notice a single cable outage, though breaks affecting multiple cables in the same region can cause measurable slowdowns or disruptions.
Who Owns Submarine Cables
The ownership model has shifted dramatically over the past decade. Submarine cables were historically built and operated by consortiums of telecommunications companies, with dozens of carriers sharing the cost and capacity of a single system. That model still exists, but the biggest new cables are increasingly funded by tech companies like Google, Meta, Amazon, and Microsoft. These companies need massive, reliable bandwidth between their data centers worldwide, and building their own cables gives them direct control over capacity, routing, and latency.
This shift has drawn scrutiny. A 2025 inquiry by the U.S. House Committee on Homeland Security pressed major tech companies on their submarine cable operations, particularly regarding potential involvement of Chinese-affiliated entities in cable maintenance and servicing. The concern centers on the fact that these cables carry virtually all international data traffic, making them critical infrastructure with national security implications. Questions about who builds, maintains, and has physical access to these systems have become a growing policy issue.
Why They Matter More Than Satellites
Satellites get more attention, but submarine cables do the heavy lifting. A single modern cable can carry hundreds of terabits per second. The entire constellation of low-earth orbit satellites currently in operation handles a fraction of that. Satellites are essential for reaching remote areas and providing backup, but for the core internet backbone connecting continents, fiber optic cables on the ocean floor remain unmatched in both capacity and cost per bit.
The physical concentration of so much global data in a few hundred cable systems also creates a vulnerability. Chokepoints where many cables converge, like the Strait of Malacca, the Red Sea, and the English Channel, represent single points of geographic risk. Natural disasters, military conflicts, or even a badly placed anchor in one of these corridors can affect connectivity for entire regions. That concentration is why governments and international bodies like the International Telecommunication Union have increasingly focused on submarine cable resilience as a matter of critical infrastructure planning.