How undersea cables carry the internet
Most traffic between continents does not go through space. It travels as pulses of light inside cables on the ocean floor. Here is how they work.
Key takeaways
Intercontinental traffic mostly travels as light in seafloor fiber cables.
Total internal reflection keeps light trapped inside the glass core.
Repeaters along the route amplify the light without converting it.
Landing stations power the cable and hand traffic to land networks.
When you video call someone on another continent, your voice almost certainly does not bounce off a satellite. It travels as pulses of light through a cable lying on the ocean floor. Several hundred of these cables link the world's coastlines, and together they carry the vast majority of intercontinental internet traffic.
Step 1: Data becomes light
At the heart of each cable are optical fibers: strands of ultra-pure glass about as thin as a human hair. Lasers at one end encode bits into the light, and modern systems vary not only its brightness but also its phase and polarization to pack in more data. Each fiber carries many colors of light at once, a technique called wavelength-division multiplexing, and every color is its own data stream.
Step 2: Total internal reflection keeps light inside
Each fiber has a glass core wrapped in cladding, glass with a slightly lower refractive index. When light in the core meets that boundary at a shallow, glancing angle, it cannot escape and reflects back inward. This effect, total internal reflection, lets light follow the fiber for thousands of kilometers, even around gentle bends.
The fibers are a tiny part of the cable. Around them sit protective tubes, steel wires for strength, a copper layer that carries power and a tough plastic jacket. Near shore, where anchors and fishing gear are a risk, cables get extra armor and are often buried in the seabed. In the deep ocean, a cable can be about as thick as a garden hose.
Step 3: Repeaters boost the signal
Even the purest glass absorbs a little light, so the signal fades with distance. Roughly every 50 to 100 kilometers along the route sits a repeater, a pressure-proof housing with optical amplifiers inside. They boost the light directly, using a section of fiber doped with the element erbium and energized by small lasers, without converting the signal back into electricity. The copper layer in the cable carries high-voltage power from shore to keep every repeater running.
Step 4: Landing stations connect to land
Each end of the cable comes ashore at a landing station. This building feeds power into the cable and houses the terminal equipment that turns the light signals back into data. From there, traffic is handed to land networks, data centers and internet exchanges, then routed like any other packet: across fiber, to a cell tower or home connection, and finally to your device.
The cloud, the web and your group chats all depend on glass on the seafloor.
Why it matters
The internet is physical. When a cable is damaged, usually by fishing gear or a ship's anchor, a specialized repair ship has to find it, lift it and splice it back together. Most routes have backup cables, which is why the majority of faults go unnoticed, but regions served by only one or two cables can lose much of their connectivity. It is also why large tech companies now fund cables of their own: whoever builds the glass shapes how quickly the world connects.
