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Not through the sky

The Internet Runs Along the Sea Floor

Almost everything you send overseas travels down a cable about as thick as a garden hose, lying on the mud at the bottom of the ocean. Satellites carry almost none of it.

The internet, aseveryone picturesit What it actually is: ahose-thick cable onthe seabed
There are well over a million kilometres of it down there.

  1. Step 1 of 5

    Satellites are a rounding error.

    Parts in ahundred oftraffic: by cableThe remainder,roughly, bysatelliteCables inservice: around600
    Roughly 99 to 1, and it has been that way for decades.

    It feels obvious that data from another continent must go up and come back down. It almost never does. Satellites handle ships, aircraft, disaster zones and remote villages — vital work, but a sliver of the total. Everything else goes through glass on the sea floor.

  2. Step 2 of 5

    The cable itself is mostly armour around a hair of glass.

    Polyethylene jacketThe outer skinSteel wire armourAgainst anchors,trawlers and rockCopper conductorPowers therepeatersPetroleum jelly and steel tubeKeeps seawateroutOptical fibresThe internet,hair-thin
    In deep water the armour is dropped entirely and the cable is no thicker than a marker pen.

    The part that carries your data is a bundle of glass fibres thinner than a hair. Everything else in the cable exists to protect them, or to feed electricity to the repeaters that boost the light every 50 to 100 kilometres along the way.

  3. Step 3 of 5

    A ship lays it, slowly, for weeks.

    1 Survey theroute firstWeeks ofsonar first2 Pay outover thesternAround 200km a day3 Bury itnear thecoastA sea ploughcuts atrench4 Deep water:just lay iton the mudNothing downthere to hitit

    The route is surveyed first to dodge canyons, volcanoes and other cables. Then a cable ship pays it out over the stern at walking pace. Near shore, a plough tows behind and buries the cable a metre or two into the seabed, because that is where the danger is.

  4. Step 4 of 5

    Light beats radio because it does not have to go anywhere near as far.

    Via geostationarysatellite ~70,000 km round trip— a quarter-seconddelayVia seabed cable ~5,500 km — under atenth of that
    Newer satellites orbit far lower and cut the delay sharply, which is why they are suddenly interesting again.

    A geostationary satellite sits 35,786 km up. A signal must climb that and come back — a round trip of over 70,000 km, which costs about a quarter of a second no matter how good your equipment is. London to New York by cable is about 5,500 km. For anything conversational, the cable wins before the technology is even considered.

    This delay is why a satellite video call has that awkward beat before the other person answers, and why traders pay enormous sums for cable routes a few hundred kilometres shorter.

  5. Step 5 of 5

    They break roughly every other day, and that is fine.

    Why an outage almostnever reaches you150 to 200 breaks a year worldwideEvery major route hasseveral separate cablesTraffic reroutes inmilliseconds

    Something like 150 to 200 faults happen each year — mostly fishing gear and ships' anchors in shallow water, occasionally an undersea landslide. Repair ships grapple the cable up, cut out the damage and splice in a new length. You never notice, because traffic reroutes through other cables in milliseconds.

The short version

Around 99 per cent of international internet traffic runs through fibre-optic cables lying on the ocean floor, because light through glass over 5,500 km beats a 70,000 km round trip to a satellite.

Try it yourself

Look up a submarine cable map online and find the cables that land nearest to you. Most coastal countries have a handful, and their landing points are ordinary-looking beaches.

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