Why 99% of the Internet Travels Through Ocean Floor Cables

If you throw a rock into the ocean off a coast in Portugal or South Africa, you’re probably chucking it near a highway. Not for cars—for cat videos. For stock trades. For your Zoom meetings. Under the surface lies a sprawling network of fiber-optic cables that carries 99% of all intercontinental data traffic. Satellites? They handle the other 1%.

So why, in an age of Starlink and 5G, are we still sending data through tubes on the ocean floor? Because physics, economics, and a little bit of Cold War history made it the only way that makes sense.

The Physics of Choosing Cables Over Space

Satellites look cool. They also suffer from latency. A signal traveling to a satellite in geostationary orbit (35,786 km up) and back takes at least 240 milliseconds. That’s fine for streaming a movie, but terrible for a high-frequency trader in London trying to beat someone in New York.

Light in glass moves slower than light in a vacuum—roughly 200,000 km/s instead of 300,000 km/s. But the route is a straight-ish line along the seabed. A transatlantic cable from New York to London has a round-trip latency of about 60 milliseconds. That’s four times faster than a satellite.

Also: bandwidth. Fiber-optic cables carry terabits of data per second. A modern cable like the 2016 MAREA (Virginia to Spain) handles 200 terabits per second. For comparison, all of Starlink’s satellites combined currently deliver about 100 terabits—to the entire planet.

So, cables win on speed and capacity. But that’s only half the story.

How They Work—It’s Simpler Than You Think

An undersea cable is basically a super-insulated hair-thin glass tube with lasers at both ends. Data is encoded as pulses of light. The glass core has a cladding around it that reflects the light inward, keeping the signal bouncing down the line without leaking out.

But you can’t just string a fiber across the Atlantic. Light attenuates—it fades. So every 50 to 100 kilometers along the route, there’s a repeater. These are cylindrical electronic boxes that look like small tuna cans. They sit on the seabed, powered by copper conductors running alongside the fiber, and boost the optical signal using erbium-doped fiber amplifiers.

No batteries. No solar panels. The repeaters are fed by high-voltage DC electricity (typically 5 to 10 kilovolts) sent from the landing stations on shore. The entire cable system is a single, fully sealed circuit. If you break it, you don’t just lose internet—you get a massive voltage spike that can fry equipment.

The Surprising Vulnerability

You’d think a cable wrapped in layers of steel, copper, and polyethylene, lying in cold water 5,000 meters deep, would be safe. It mostly is. But ships dragging anchors and fishing trawlers using bottom trawls are the top cause of cable damage. A 2022 study from the International Cable Protection Committee found that about 80% of cable faults are caused by human activity—primarily fishing and anchoring.

Shark bites also happen. Not because sharks are cyber-attackers, but because the cables emit a small electromagnetic field that mimics the bioelectric field of prey. One bite, a few punctures, and an entire transatlantic link goes dark.

Repairs are not quick. A repair ship has to locate the exact break point, which is usually found by sending a laser pulse down the fiber and measuring the reflection time (optical time-domain reflectometry). Then they drop a grapnul, pull the cable up to the surface, splice in a new segment, and lower it back. This takes days to weeks.

The Geopolitical Monopoly

Don’t look at the map of undersea cables and think it’s a free market. It’s a map of power. Historically, cables land only in countries that are politically stable and willing to play nice with the major cable operators—mostly Google, Meta, Amazon, Microsoft, and a few telecom giants like SubCom and Alcatel Submarine Networks.

China? It routes a huge share of its traffic through cables that land in Hong Kong and Shanghai, but also through cables that pass near countries like Vietnam and the Philippines. The U.S. and allies have repeatedly expressed concerns that Chinese-built or Chinese-controlled cables could be tapped or rerouted in a conflict. The 2013 PRISM revelations showed that the U.S. National Security Agency was already tapping cables landing in the U.K. and the U.S.

So cables are not neutral. They’re infrastructure built by corporations, defended by navies, and surveilled by intelligence agencies.

The Unlikely Role of Satellites

That 1% that satellites carry? It’s not just for yacht Wi-Fi. Satellites are a backup. When a major cable goes down—like the 2022 Tonga volcanic eruption that severed the single cable serving the island nation—satellites provide a lifeline. Starlink’s low-Earth-orbit constellation has actually become a critical emergency network for remote regions and disaster zones.

But Starlink can’t scale to replace the cable network. A single cable landing station in New Jersey can handle as much data as every Starlink satellite combined. And even if you launched 100,000 more satellites, latency would always be worse than glass because physics doesn't negotiate.

What Happens If They All Break?

Let’s say a massive solar flare or a coordinated attack took out a dozen major cables simultaneously. The internet wouldn’t “go dark” globally—but it would become regionally fragmented. Global latency would spike. Streaming services in Europe would lose access to U.S. content databases. Financial markets would halt cross-border trades. Blue screen of death? No. A slower, yellower, much angrier internet.

Countries like Norway and Japan have already started building “cable fleets”—multiple diverse routes so that if one cable is cut, traffic instantly reroutes through others. But the system has weak points. The Suez Canal is one. The Luzon Strait off Taiwan is another. A cable cut in a chokepoint can take out huge amounts of traffic simultaneously.

The Takeaway

The internet isn’t a cloud. It’s a wet, cold, fragile glass tube running along the bottom of the ocean, constantly being nibbled by sharks and dragged by fishing boats. And that’s the part we don’t see.

Next time someone talks about satellite internet “replacing” fiber, remind them: 99% of this conversation is traveling through a cable you couldn’t reach with a submarine, held together by copper and hope, patrolled by navies, and repaired by a small number of specialized ships. It works because it has to. There’s no Plan B.