Undersea Cables: The Hidden Network That Carries 99% of Internet Traffic

1.4 Million Kilometers Beneath the Ocean

Over 500 submarine cables crisscross the world's ocean floors, carrying approximately 99% of all intercontinental data traffic. These fiber-optic lines handle everything from financial transactions and video calls to streaming media and cloud computing. Satellites, despite their visibility, carry less than 1% of international internet traffic. The global internet is, at its core, a network of wet cables.

Total cable length exceeds 1.4 million kilometers — enough to circle the Earth more than 35 times.

Anatomy of a Submarine Cable

A modern submarine cable is surprisingly thin. In deep water, the cable is roughly the diameter of a garden hose — about 17 millimeters. Near shore, where anchors and fishing trawls pose threats, armored versions can be several centimeters thick.

Each cable contains multiple fiber pairs. Modern systems carry 16 or more fiber pairs, and each fiber can transmit data at speeds exceeding 20 terabits per second using wavelength-division multiplexing — splitting a single fiber into dozens of independent channels of different light wavelengths.

Laying Cable Across Ocean Basins

Cable-laying ships are specialized vessels costing hundreds of millions of dollars. Only about 40 exist worldwide. The process is slow and precise.

• Route surveys map the ocean floor using sonar, identifying hazards like submarine volcanoes, canyons, and fault lines
• The cable is loaded onto the ship's holds in a continuous coil — some ships carry over 8,000 kilometers of cable in one load
• A plow buries the cable 1–2 meters below the seabed in shallow waters to protect against anchors and trawling
• In deep ocean (below about 1,500 meters), the cable simply rests on the seabed — burial is unnecessary because human activity does not reach those depths

A transatlantic cable installation typically takes several months from start to finish. The route planning alone can take years.

Repeaters: Amplifying Light in the Deep

Optical signals weaken over distance. Repeaters — sealed, pressurized units spaced roughly every 60–100 kilometers along the cable — amplify the light using erbium-doped fiber amplifiers. A single transatlantic cable may contain over 100 repeaters, each powered by electrical current sent through the copper conductor from shore stations. Repairing a failed repeater requires retrieving it from the ocean floor.

Threats and Vulnerabilities

Despite their importance, submarine cables face constant physical threats.

Cable faults occur roughly 100–200 times per year worldwide. Repair ships must sail to the fault location, grapple the cable from the seabed (sometimes from depths exceeding 4,000 meters), splice in a new section, and relay the cable. Repair can take two to four weeks depending on location and weather.

Geopolitics of the Cable Map

Submarine cables are strategic assets. Nations with many cable landings have resilient internet connectivity. Island nations or countries with few landing points are vulnerable to isolation if cables fail.

• The United States has over 90 submarine cable landings, providing massive redundancy
• Tonga lost nearly all internet connectivity for five weeks in 2022 when a volcanic eruption severed its single international cable
• Content companies (Google, Meta, Microsoft, Amazon) now own or co-own a growing share of new submarine cables, shifting control from telecom carriers to tech platforms
• China's HMN Technologies (formerly Huawei Marine) has become a major cable manufacturer, raising security concerns among Western governments

The concentration of cable routes through narrow chokepoints — the Strait of Malacca, the Red Sea, the English Channel — creates geopolitical vulnerability. A natural disaster or conflict in these areas could disrupt global communications.

Capacity Growth and Future Technology

Demand for submarine cable capacity roughly doubles every two to three years. Engineers meet this demand through several innovations: spatial division multiplexing (using more fiber pairs per cable), improved signal processing that pushes each fiber closer to its Shannon limit, and hollow-core fibers that reduce signal latency by transmitting light through air instead of glass.

The newest generation of cables connects data centers directly rather than following traditional telecom routes between population centers. This reflects the shift in internet architecture from voice-centric telecommunications to cloud-centric data services. The invisible backbone of the global internet remains, quite literally, grounded on the ocean floor.