How the internet crosses the ocean

1. Your data becomes light

Your data starts as electrical signals. In the landing station, optical terminal equipment encodes it onto a laser carrier. Modern coherent transmission uses changes in the optical field, including amplitude and phase, rather than simply switching a laser on for each one and off for each zero.^[6]

The scenes below move from representative hardware into explanatory cutaways. Visible colours reveal infrared signals we cannot see with our eyes. Camera motion and signal timing are slowed for inspection; the source notes identify where construction is based on a specific published design.

2. Light trapped in glass

An optical fibre has a higher-index glass core surrounded by lower-index glass cladding, which confines the guided light. Long-distance telecommunications generally use single-mode fibre. Our wave views show the optical field; they are not a literal ray zigzagging through a hollow tube.^[9]

Light in glass travels at about 200,000 km a second, two-thirds of its speed in a vacuum.^[5] That’s why distance matters: crossing the Atlantic on about 6,000 km of cable takes around 30 milliseconds each way, before any equipment adds its own delay.

3. Many colours in one fibre

One fibre carries many beams at once, each a slightly different colour (wavelength), each its own channel. This is called wavelength-division multiplexing.^[3] Add more fibres to the cable and the capacity multiplies again: a modern cable such as MAREA, between Virginia and Spain, can carry 224 terabits (224 million megabits) per second.^[1]

4. What the cable looks like

In the deep ocean a cable is about as wide as a garden hose,^[1] roughly 25 mm.^[2] Construction varies. The cutaway below follows the published OCC-SC300 example: loose fibres and water-blocking compound, a segmented steel tube, strength wires, a copper tube and polyethylene insulation. Additional armour is used where more protection is needed.^[7]

5. Boosting the light

Even in the purest glass, light slowly fades. So a long cable has repeaters on the seabed along its length; fibre allows runs of more than 100 km between them,^[2] so an ocean crossing needs dozens. Inside a sealed repeater, pump lasers supply energy to erbium-doped fibre to amplify the passing optical signal. The data stays optical; amplification also adds noise.^[8]

The repeaters need power, and there’s no socket on the seabed. Equipment at the landing stations sends a direct current down the cable’s copper layer, often at 3,000 to 15,000 volts.^[2]

6. Coming ashore

At each end the cable comes up a beach to a landing station, where the light is turned back into data and handed to the networks on land: internet providers, data centres, and from there to you. Nearly every country with a coastline is connected to at least one cable.^[1] Our map shows 1,927 landing points in 186 countries.

7. When a cable breaks

There are roughly 200 cable faults a year worldwide. About two-thirds are accidents: fishing gear and ships dragging their anchors. Earthquakes and underwater landslides cause some too; sharks, despite the stories, don’t matter.^[1]

1. 1Traffic moves. Networks reroute over other cables, so most people notice little more than a slower connection, unless a place depends on just one or two cables.
2. 2The break is found. Engineers measure from the shore how far along the cable the fault is.
3. 3A repair ship goes out. It lowers a grapnel, a hook dragged along the seabed, catches the cable and lifts it to the deck, one end at a time in deep water.^[2]
4. 4New cable is spliced in. The damaged piece is cut out, a new section is joined to both ends, tested, and lowered back down.^[2]

A repair can take days or weeks, depending on the weather, the depth, permits, and whether a ship is free. Across the faults we’ve recorded, the typical outage has lasted a few weeks. Follow them on Incidents, and the cable ships that fix them.

8. What about satellites?

Satellites matter for remote places, ships and planes, but they carry a tiny share of international traffic: just 0.37% of the United States’ international capacity. Cables carry far more data at a fraction of the cost.^[1]