Almost all of the internet travels through cables lying on the ocean floor

Satellites get most of the popular imagination when people picture how a video call crosses an ocean or an email reaches another continent, but the physical reality is far less glamorous. The overwhelming majority of international data, from bank transfers to streaming video to military communications, still travels through fiber-optic cables laid directly on the seafloor, a network most people never see and rarely think about until one gets cut.

A backbone most users never notice

A submarine communications cable is a bundle of optical fibers, insulated and armored, laid across ocean and sea beds to link land-based stations on different continents. Modern versions carry telephone traffic, internet data and private corporate networks alike, transmitting signals as pulses of light rather than electricity. Satellites, by contrast, handle only a small fraction of intercontinental traffic, largely because they carry far less bandwidth per connection and introduce a noticeable delay from the signal’s round trip to orbit and back. Estimates for the undersea share of international data traffic run as high as roughly 95 to 99 percent, meaning a phone call or webpage crossing between continents almost always rides fiber lying on the ocean floor rather than beaming through space.

A lineage that starts with copper and telegraphs

The idea of wiring continents together predates the internet by more than a century and a half. The first attempt at a transatlantic telegraph cable, backed by the Atlantic Telegraph Company under entrepreneur Cyrus Field and engineers John Watkins Brett and Charles Tilston Bright, went into the water in 1857 using two repurposed naval ships, HMS Agamemnon and USS Niagara. That attempt failed when the cable snapped, and a version that briefly worked in 1858 died out within weeks. According to IEEE Spectrum’s account of the early transatlantic effort, a durable connection did not arrive until 1866, when the converted passenger liner Great Eastern successfully laid a cable that held. That ship went on to repair and lay additional undersea telegraph lines into the 1880s, establishing the basic model — a purpose-built cable ship, a grappling hook to retrieve broken lines, and onboard splicing — that the industry still uses in modified form today.

From telegraph pulses to light in glass

Copper telegraph and telephone cables eventually gave way to fiber optics in the late 20th century, a shift that multiplied capacity by orders of magnitude. Where early cables carried a handful of telegraph messages, a modern fiber pair can carry many terabits of data per second, and a single cable system typically bundles multiple fiber pairs together inside protective layers of steel wire, polyethylene and, in shallow coastal waters, extra armoring against fishing trawlers and ship anchors. Repeaters spaced at intervals along a route boost the optical signal so it does not degrade over transoceanic distances, a challenge telegraph engineers never had to solve since their signal was a much simpler electrical pulse.

A physical map larger than most people expect

The scale of the current network is easy to underestimate because so little of it is visible. According to a detailed survey of submarine communications cables, more than 500 submarine cable systems are in service worldwide, stretching across roughly 1.3 million kilometers of ocean floor in total, linking every inhabited continent and most island nations to the rest of the global network. Cables land at specific coastal stations, then connect inland to the data centers and internet exchange points that route traffic the rest of the way, meaning a single stretch of undersea fiber can end up carrying a meaningful share of the connectivity for an entire country or region.

Why a fishing trawler can cause a continental outage

Because the cables are physical objects lying exposed or shallowly buried on the seabed, they remain vulnerable to the same hazards that threatened 19th-century telegraph lines: ship anchors, fishing gear, underwater landslides and, closer to shore, simple accidental snagging. Operators respond to breaks the same way their 1858 predecessors did, sending a specialized cable ship to locate the fault, haul the cable to the surface and splice in a new section, though modern vessels use remotely operated submersibles and precise fault-location electronics rather than a dragged grappling hook. Redundancy across multiple routes is the main defense against any single cut causing a total blackout, which is why major cable operators and governments increasingly treat the layout of this undersea map as a strategic infrastructure question rather than a purely commercial one.

Who owns the pipes, and why redundancy matters most

Ownership of this infrastructure has shifted dramatically from its telegraph-era origins. The earliest transatlantic lines were built and controlled by national telegraph companies and, later, government-linked telephone monopolies that treated the cables as extensions of state communications policy. Today a large and growing share of new cable capacity is financed directly by large technology and cloud-computing companies rather than traditional telecom carriers, since those firms move enormous volumes of data between their own data centers across continents and have found it cheaper to fund dedicated cables than to lease capacity from others. That shift has changed who decides where new routes get built, but it has not changed the underlying physics or vulnerability of a cable resting on the ocean floor, which is why the ownership structure of individual routes has become a subject of national security scrutiny in several countries with major landing stations.

Because any single cable can be cut, whether by accident or deliberate sabotage, the resilience of the whole system depends on how many independent paths exist between two points rather than on any one line’s durability. Regions served by only one or two cable routes, common among smaller island nations, can lose the bulk of their international connectivity from a single break, sometimes for days or weeks until a repair ship reaches the site, while regions crossed by dozens of parallel cables barely register the loss of any individual line. That uneven distribution of redundancy is a large part of why cable route planning has become as much a geopolitical exercise as an engineering one, with new lines increasingly designed to avoid chokepoints where too much of a region’s traffic would otherwise funnel through a single vulnerable stretch of seabed.

This article was produced with the assistance of AI and reviewed by an editor.

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