Submarine Cable

When you stream a foreign video on your phone or load a website hosted halfway across the world, the data does not beam through thin air. Instead, it rushes across the globe in a fraction of a second through slender lines quietly resting on the seafloor.

Packed inside each submarine cable are bundles of hair-thin glass fibers. By pulsing laser light down the core of these glass strands, light bounces continuously off the inside walls (a process called total internal reflection) to carry vast amounts of data at incredible speeds.

Thanks to fiber-optic technology, even high-definition 4K videos and massive photo galleries cross oceans with almost imperceptible delay. The global internet we rely on every day is really powered by an enormous underwater mesh of physical cables.

Many people assume our international internet beams from satellites orbiting high above Earth. In reality, subsea cables handle over 95 percent of all international data traffic.

The primary reason is sheer speed and capacity. Traditional geostationary satellites sit roughly 22,000 miles (36,000 kilometers) above Earth. Signals must make a long round trip, creating noticeable lag, and bad weather can disrupt the feed. While newer low Earth orbit (LEO) satellites offer lower latency, they still cannot match the bandwidth or cost-efficiency of physical cables. Subsea cables travel straight across the seabed without detouring into space, delivering much lower latency than geostationary satellites.

On top of that, a single undersea cable can carry more data than dozens of satellites combined. Laying cables across the seabed remains far cheaper, more reliable, and vastly higher-capacity than launching swarms of satellites into orbit.

If you just tossed bare wires into the ocean, passing ship anchors and commercial fishing nets would shred them in no time. In fact, most cable faults are caused by fishing gear and dropped anchors, with rare disruptions from underwater landslides, earthquakes, or marine life. That is why the fragile glass fibers at the core are wrapped in multiple protective layers of high-tensile steel wire, waterproof plastic, and copper tubing.

Cable designs actually vary based on water depth. In shallow coastal waters, where ship traffic is heavy and trawling nets scrape the bottom, thick, heavily armored cables—about as thick as a person's forearm—are buried beneath the seabed. Out in the deep abyss thousands of meters down, where hazards are scarce, much thinner cables no thicker than a garden hose simply rest directly on the ocean floor.

Because optical signals naturally fade over thousands of miles, engineers install submerged repeaters every few dozen miles to amplify the light. When a cable does break, specialized repair ships race out to hook the severed line from the depths, pull it to the surface, and splice each microscopic fiber by hand.