Undersea Cables

Every international email you send, every video call to another continent, every cloud backup stored overseas—right now, that data is traveling as pulses of light through glass fibers thinner than a human hair, inside cables lying in total darkness on the ocean floor.

Over 95% of international Internet traffic flows through undersea cables. Not satellites. Not some ethereal "cloud." Physical cables, spanning thousands of miles across the world's oceans, connecting continents with fiber optic strands we can't see and rarely think about.

This is the actual backbone of the global Internet.

What These Cables Actually Are

A modern undersea cable is surprisingly modest in appearance. In the deep ocean, it's about the diameter of a garden hose. You could wrap your hand around the infrastructure carrying millions of simultaneous video streams.

Inside that modest exterior:

• Optical fibers at the core—hair-thin strands of ultra-pure glass transmitting data as light
• Copper sheathing that powers the amplifiers along the route
• Steel armor and waterproof insulation for protection
• Multiple fiber pairs—modern cables contain dozens to hundreds, each carrying many wavelengths of light simultaneously

Near coastlines, where anchors and fishing equipment pose threats, cables bulk up with heavy armor. But in the deep ocean, where the only dangers are geological, they're remarkably thin for what they carry.

The longest cables exceed 12,000 miles. The most advanced carry over 300 terabits per second—enough for millions of HD video streams simultaneously. They cost hundreds of millions to over a billion dollars to build and deploy.

How Light Crosses Oceans

Light weakens as it travels through fiber. After about 50 miles, the signal has degraded enough to need help. So every 30-50 miles along the cable, there's a repeater—a sealed unit containing optical amplifiers that boost the signal and send it onward.

A transatlantic cable might contain over 100 repeaters, each one powered by electrical current flowing through the copper sheathing from shore stations at either end. These repeaters sit on the ocean floor, in crushing pressure and total darkness, working continuously for decades.

When the cable reaches land, it terminates at a landing station—a facility that connects the submarine cable to terrestrial networks. Landing stations supply power to all those repeaters, convert signals between submarine and land-based formats, and monitor the entire cable for faults. From there, traffic flows onto land-based fiber networks, eventually reaching ISPs, data centers, and your device.

Where the Cables Run

Cable routes are chosen carefully:

• Shorter is better—less cost, lower latency
• Avoid hazards—seismic zones, deep trenches, heavy shipping lanes, fishing grounds
• Connect economic centers—cables go where the money and people are
• Follow proven paths—new cables often parallel existing routes where seabed conditions are known

The major routes:

Transatlantic: Dozens of cables connect North America and Europe, carrying enormous traffic between these economic centers.

Transpacific: Some of the world's longest cables link North America to Asia across the Pacific.

Europe-Asia: Cables thread through the Mediterranean, Red Sea, and Indian Ocean.

Regional networks: Cables connect Asian countries to each other, link Pacific islands, run along the Americas' coastlines.

Look at a submarine cable map and you see the physical shape of the global economy—thick bundles connecting wealthy regions, thinner strands reaching developing areas, strategic chokepoints where geography forces cables through narrow passages.

Who Owns the Ocean's Internet

Ownership has shifted dramatically.

Traditionally, telecommunications companies formed consortiums. Multiple carriers would fund a cable together, each receiving a share of capacity. This spread the enormous costs and risks.

Now, increasingly, the cables belong to Google, Meta, Amazon, and Microsoft. These companies need massive international bandwidth for their services. They discovered it's cheaper to own the infrastructure than to keep buying capacity from telecom companies. Google alone owns or co-owns cables spanning multiple oceans.

This shift matters. The physical infrastructure of international Internet connectivity is increasingly controlled by a handful of tech giants rather than regulated telecom carriers.

Laying Cable Across an Ocean

Building an undersea cable is one of the largest engineering projects humans undertake:

Survey: Ships spend months mapping the route with sonar, identifying every hazard, planning the exact path the cable will follow.

Manufacturing: Factories produce thousands of miles of cable, wound onto massive spools. The cable ships themselves are specialized vessels with enormous storage holds.

Deployment: The ship travels the route slowly, paying out cable continuously. In shallow water, plows bury the cable in the seabed. In the deep ocean, cable simply lies on the bottom—at those depths, there's nothing to disturb it.

Testing: Once laid, every fiber is tested, every repeater verified. Only after extensive testing does the cable enter service.

A transatlantic installation takes several months. The logistics are immense—coordinating with maritime traffic, managing weather windows, ensuring the ship's cable supply matches the remaining route distance.

When Cables Break

Cables fail. Not often, but it happens:

• Fishing trawlers and ship anchors in shallow water (the most common cause)
• Earthquakes and underwater landslides
• Turbidity currents—underwater avalanches of sediment
• Shark bites (rare, but documented—sharks seem attracted to the electromagnetic fields)
• Repeater failures

When a cable fails, landing stations detect it immediately. By analyzing signal characteristics, technicians can estimate where the fault occurred.

Repair involves sending a cable ship to the location. In deep water, a grappling hook snags the cable from the seabed. The ship hauls up both ends, cuts out the damaged section, splices in new cable, and lowers it back down. This can take days or weeks depending on location, weather, and damage severity.

During repairs, traffic reroutes through other cables. This is why major routes have redundant cables—losing one shouldn't cut off connectivity entirely.

Why Governments Care

Undersea cables have become strategic infrastructure:

Intelligence: Governments recognize that cables carry vast amounts of communications. Some have developed capabilities to monitor cable traffic, raising significant privacy questions.

Economic dependence: A country's cable connectivity directly affects its ability to participate in the digital economy. Poor connectivity means competitive disadvantage.

Geopolitical tension: Cable routes reflect political relationships. Some nations specifically build cables to avoid routing through rivals' territories.

Vulnerability: Cables are potential targets in conflicts. Their routes are publicly known. Severing key cables could severely disrupt a nation's international connectivity.

Awareness of these vulnerabilities is driving investment in redundant routes and diverse paths. Some worry this could fragment the Internet as nations build parallel infrastructure to reduce dependence on cables they don't control.

What About Satellites?

Satellite Internet gets attention, but cables carry the load:

Capacity: A single modern cable carries more data than all communications satellites combined

Latency: Geostationary satellites add 500+ milliseconds of delay; cables add single-digit milliseconds

Cost: Cable bandwidth costs a fraction of satellite bandwidth per bit

Reliability: Cables work regardless of weather

Satellites matter for remote areas, ships, aircraft, and backup connectivity. But for the heavy lifting of international Internet traffic, there's no alternative to cables. The physics simply favor glass fibers on the ocean floor over radio waves bouncing off satellites.

What's Coming

More capacity: New technologies like space-division multiplexing will increase capacity without laying new cables.

Arctic routes: Melting ice is opening paths between Europe and Asia that are thousands of miles shorter than current routes.

Broader reach: New cables increasingly target underserved regions—Africa, Pacific islands, developing nations gaining their first high-capacity international links.

Continued consolidation: Tech giants will likely keep dominating new cable investments, further concentrating control of this infrastructure.

The Invisible Foundation

We talk about the Internet as if it floats somewhere above us—the cloud, the ether, the digital realm. But the Internet is physical. It's glass and copper and steel, lying on the ocean floor in darkness and pressure, carrying light pulses that somehow become your video calls and emails and streaming movies.

The next time you connect to something overseas, remember: your data is traveling through a cable thinner than a garden hose, maintained by repeaters that have been working in the deep ocean for years, following routes first established when humans were still figuring out how to send telegrams across the Atlantic.

The technology has changed. Light instead of electricity. Terabits instead of words per minute. But we're still stretching cables across oceans, still trusting thin strands of material to carry our communications between continents.

It works remarkably well. And almost nobody thinks about it.