The Undersea Web: How Submarine Cables Hold the Internet Together

Somewhere beneath the Atlantic Ocean, roughly 3,000 meters below the surface, a glass fiber thinner than a human hair is carrying a video call between London and New York. Next to it, bundled into a cable no wider than a garden hose, dozens of identical fibers are transmitting financial transactions, streaming video, government communications, and billions of emails — all at the speed of light. The cable rests on the ocean floor, buried in sediment where possible, exposed on rocky seabeds where it is not. It has been there, in some form, since the first telegraph cables were laid in the 1850s.

This is the physical internet. Not the cloud, not the satellites, not the wireless signals that seem to conjure data from thin air. Approximately 99 percent of all intercontinental data travels through submarine cables — more than 500 of them, stretching over 800,000 miles across every ocean on earth.

From Telegraph Wire to Fiber Optic

The story of undersea cables begins with the transatlantic telegraph. In 1858, after several failed attempts, a cable was successfully laid between Ireland and Newfoundland. The first message, from Queen Victoria to President James Buchanan, took 16 hours to transmit. The cable failed after three weeks, but the principle had been proven: the ocean floor could serve as a conduit for communication.

By the 1870s, the British Empire had built a global telegraph network, connecting London to India, Australia, and the Americas. The cables were copper, the signals analog, the capacity limited. But the infrastructure laid down in the 19th century established routes and landing points that are still used today.

The transition to fiber optics in the 1980s transformed the industry. A single modern fiber optic cable can carry tens of terabits per second — enough to transmit the entire contents of the Library of Congress in a matter of seconds. The cables themselves are marvels of engineering: glass fibers surrounded by layers of steel wire, copper sheathing, and polyethylene insulation, designed to withstand the crushing pressure of the deep ocean.

"People think the internet is in the sky — in satellites, in the cloud. In reality, it is at the bottom of the ocean, held together by cables not much thicker than your thumb."

Who Builds and Owns the Cables

For most of the 20th century, submarine cables were built and operated by telecommunications consortia — groups of national carriers that shared the cost and capacity. That model has changed dramatically in the past decade. Today, the largest investors in submarine cable infrastructure are technology companies: Google, Meta, Microsoft, and Amazon.

Google alone has invested in or fully owns more than two dozen submarine cable systems. Its Dunant cable, completed in 2020, connects the United States and France with a capacity of 250 terabits per second. Meta's 2Africa cable, one of the longest ever built, encircles the African continent with 45,000 kilometers of fiber, connecting it to Europe and the Middle East.

This shift in ownership has raised questions about concentration and control. When a handful of private companies own the physical infrastructure that carries the world's data, the implications for competition, access, and sovereignty are profound. Smaller nations and telecom operators increasingly depend on capacity leased from these tech giants, creating new forms of digital dependency.

Vulnerabilities: Anchors, Earthquakes, and Espionage

Despite their importance, submarine cables are remarkably vulnerable. The most common threat is accidental damage from ship anchors and fishing trawlers, which account for roughly two-thirds of all cable faults. In 2006, the Hengchun earthquakes off the coast of Taiwan severed multiple cables simultaneously, disrupting internet service across Southeast Asia for weeks.

Natural disasters are not the only concern. Submarine cables have long been targets of espionage. During the Cold War, the U.S. Navy conducted Operation Ivy Bells, tapping Soviet undersea communication cables in the Sea of Okhotsk. In the modern era, concerns have shifted to potential sabotage. NATO has warned repeatedly about Russian submarine activity near critical cable routes in the North Atlantic, and several unexplained cable cuts in northern Europe in recent years have heightened anxiety about deliberate interference.

Repair is slow and expensive. Specialized cable ships, of which there are fewer than 60 in the world, must sail to the fault location, grapple the cable from the ocean floor, splice the break, and relay it — a process that can take weeks depending on depth and weather conditions.

Geopolitics on the Ocean Floor

The routing of submarine cables has become a geopolitical concern of the first order. The United States has actively pressured allies to exclude Chinese companies, particularly Huawei Marine (now HMN Technologies), from cable-building projects, citing security risks. In response, China has accelerated its own submarine cable investments, particularly in the South China Sea and along its Belt and Road corridors.

Island nations in the Pacific, long dependent on a single cable for their internet connectivity, have become sites of strategic competition. When the tiny nation of Nauru needed a new cable connection, both the United States and China competed to provide it — not because of the revenue involved, but because of the strategic implications of controlling the digital link.

The Fragile Backbone

The submarine cable network is, by almost any measure, the most critical infrastructure that most people have never heard of. It carries global finance, military communications, diplomatic traffic, and the daily digital lives of billions of people. Yet it remains lightly regulated, unevenly protected, and poorly understood by the public.

As demand for bandwidth continues to grow — driven by cloud computing, artificial intelligence, and the ever-expanding appetite for streaming media — the network will need to expand. New cables are being planned across the Arctic, through previously ice-covered routes opened by climate change. Others will connect underserved regions of Africa, Southeast Asia, and the Pacific Islands.

The internet's future, like its present, will be written not in code but in glass and steel, laid carefully along the ocean floor by ships that most people will never see. The infrastructure is invisible, but the consequences of its failure — or its control by the wrong hands — would be felt by everyone.