How are deep-sea cables laid?

It’s a serious undertaking to connect each country to the internet across thousands of kilometres of ocean

(Image source: Future PLC/ Ed Crooks)

We often think we’re living in an increasingly wireless world. We store our data in the cloud and view it on mobile devices that we can carry around in our pockets. But phone masts that give us 4G signal are just the tip of the internet iceberg. More than 95 per cent of international web traffic is actually carried by undersea cables.

Currently there are approximately 378 of these telecommunication tentacles crisscrossing the seafloor. With a combined length of roughly 1.2 million kilometres, they link the US to Europe, France to India and more. In fact, they connect every continent except Antarctica.

Submarine cables aren’t new. Samuel Morse, who developed the telegraph, successfully sent his dots and dashes via New York Harbor in 1842. But it was the UK that saw the potential in this technology, using it to connect its vast colonial empire. The first transcontinental cable linked Ireland to Newfoundland in 1858.  Within 14 years, London could send a message to New Zealand via Bombay, Singapore and China. In 1892 British companies controlled two-thirds of the telegraph networks worldwide.

While the telegraph is no more, undersea cables have not only survived but thrived. The earliest intercontinental line could only send a few words an hour. But in February 2019 the MAREA cable, stretching almost 6,500 kilometres from Virginia Beach in the US to Bilbao, Spain, successfully generated signal speeds of 26.2 terabits per second. That’s like streaming 7 million HD movies at once. This is thanks to fibre optic cables.

Carrying pulses of light fired by powerful lasers, which can then be decoded as data, these cutting-edge cables send information at incredible speed. Today companies like Google, Facebook and Amazon are spending billions of dollars on infrastructure to boost their bandwidth, by laying their own fibre optic cables across the world’s oceans.

Laying the cables

Setting sail

A ship trails a vast length of cable from a landing station out to sea, while balloon buoys prevent the cable from being damaged or sinking.


As the cable ship gets further out to sea, the buoys are removed and the cable sinks below the water.

A long voyage

After travelling great distances to reach a second station or a designated point mid-ocean, the ship meets another cable.

Making connections

A specialist on-ship ‘jointer’ carefully splices the two together, then the conjoined cable is released and buried beneath the seafloor.

(Illustrations by Ed Crooks)

This article was originally published in How It Works issue 126, written by Jack Parsons

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