When you send a message to someone on another continent, the journey usually does not involve a satellite. It goes through a cable lying on the ocean floor. According to the National Oceanic and Atmospheric Administration, more than 95 percent of international data and voice traffic travels by fiber-optic submarine cables. These cables are typically no wider than a garden hose in the deep ocean, yet a single one can carry an immense share of the traffic between continents.
The reason is capacity and delay. Glass fibers carry vastly more data than a radio link, and a signal moving through them takes far less time to cross an ocean than one that must travel up to a satellite and back. Building and maintaining such links is a remarkable mix of optics, power engineering, and ship operations.
What Is Inside a Cable
A modern submarine cable is a layered structure. At its heart is a small set of optical fibers, often protected inside a metal or plastic tube. Around them lies a conductor, usually copper, that carries electric current. Insulating plastic seals the assembly against seawater, and in shallow waters, where the risk of damage is highest, steel wire armor is wrapped around the outside. In the deep ocean, where nothing disturbs the cable, the design is lighter.
The fibers themselves work on the principle explained in how fiber optics carry information as light: pulses of infrared laser light are confined in an ultrapure glass core. A cable typically holds a limited number of fiber pairs, and each pair can carry many separate colors of light at once, so total capacity depends more on the equipment at each end than on the cable's thickness. TAT-8, the first transatlantic fiber cable, had two working pairs of fibers.
Keeping the Light Alive Across an Ocean
Even the clearest glass weakens light, and an ocean is thousands of kilometers wide. So cables include repeaters, sealed pressure-resistant units spaced along the route every several tens of kilometers. TAT-8 used repeaters that converted light to electricity, regenerated the signal, and converted it back. Later systems use optical amplifiers that boost the light directly.
Repeaters need electric power, and there are no outlets on the seabed. The conductor in the cable carries a steady current from power feeding equipment at the landing stations on shore, often at high voltage, to run everything along the route. Repeaters therefore have to be extremely reliable, because a failure at the bottom of the ocean means dispatching a ship. The fibers carry data; the copper carries power.
How Cables Are Laid
Specialized cable ships load thousands of kilometers of cable into huge tanks and pay it out steadily as they cross a carefully surveyed route. Surveys identify smooth, stable seabed and avoid steep slopes, active fault zones, and areas where anchors or trawlers are common. Near shore, where hazards are greater, cables are often buried by plows towed along the seabed or by remotely operated vehicles.
The route ends at a landing station, a secure facility on the coast where the cable's signals connect to terrestrial networks and where the power feeding equipment sits. From there, data flows onward through land fibers toward data centers and, ultimately, toward the cloud services many people use daily.
A Long History
The idea is older than the internet. The first undersea telegraph cable crossed the English Channel in 1850. A transatlantic telegraph cable operated briefly in 1858 but failed within weeks, and a durable link was established in 1866, transmitting about twelve words per minute. Telephone service across the Atlantic came in 1956 with coaxial cables linking London and North America.
The optical era began on December 14, 1988, when TAT-8 entered service. Built by a consortium including AT&T, France Télécom, and British Telecom and costing about US$335 million, it carried 40,000 simultaneous telephone calls, roughly ten times the capacity of the last copper cable. It also faced an unexpected problem: sharks that chewed through the plastic covering of experimental cable near the Canary Islands. A privately financed competitor, PTAT-1, followed in 1989 with much higher capacity, and successive systems have grown rapidly since. International treaties governing cable protection date back to 1884.
Why Cables Beat Satellites for Bulk Traffic
A satellite in geostationary orbit sits about 36,000 kilometers above Earth, so a signal makes a round trip of roughly 72,000 kilometers before any processing, adding a noticeable delay. A fiber path between continents might be 6,000 to 10,000 kilometers long, and light in glass covers about 200,000 kilometers per second, so the trip takes tens of milliseconds. Capacity also favors fiber: each cable can carry a volume of traffic that a single satellite could not match. Satellite links remain valuable for ships, aircraft, remote regions, and broadcasting, where laying a cable is impractical.
Limitations and Vulnerabilities
Cables can be damaged, and they are. Most faults are caused by fishing gear and ship anchors near shore; earthquakes, underwater landslides, and wear can also break them. The internet copes because networks have redundancy: multiple cables serve most routes, and traffic can be rerouted when one fails. Some places, especially small islands with only one or two connections, are more exposed.
Cable faults are usually fixed at sea, not from shore, and each repair is a major logistical operation. Repairs require a ship. Crews locate the fault by measuring how light reflects back inside the fibers, retrieve the cable from the seabed, cut out the damaged section, splice in new cable, and lower the link back down. Repair time depends mainly on ship availability, distance, weather, and permits.
Security is another concern. The light in the fibers is not readable from outside the cable without physically tapping it, but operators rarely rely on that alone. The data is typically protected with the methods in how encryption protects information, and international treaties and national laws regulate cable laying and repair.
Common Misconceptions
A common belief is that satellites carry most internet traffic. They do not; they carry a small fraction of it. Another is that cables are fragile lines lying loose on the ocean floor everywhere. In reality, they are surveyed, often buried near shore, and monitored. Nor are the cables owned by a single body: many are financed by consortia of telecommunications carriers, and increasingly by large technology companies, sometimes as private ventures. And wireless links like Wi-Fi are almost always just the last few meters of a path that eventually runs through fiber, and often through an undersea cable.
In Short
Undersea cables are layered lines of ultrapure glass, protected by insulation and armor and powered from shore-based stations. Repeaters or amplifiers refresh the light along the way, ships lay and repair them, and their capacity and low delay explain why they, not satellites, carry the great majority of intercontinental data.
