The cables under the sea

Some nights the AI app takes ages to answer. Your phone shows full signal. Your data pack is fresh, and other apps on the phone open fine. The problem may be nowhere near you. It may be a broken cable lying on the floor of an ocean.

Almost none of it goes through the sky

Most people picture the internet going up. A signal leaves the phone, touches a satellite, and comes down in another country. That picture is wrong for nearly everything you do.

Data that crosses an ocean almost always goes under it. More than 99 parts in 100 travel through cables resting on the sea floor. Your search, your video call and your AI answer all take that road.

These cables are counted, not guessed. On 2 July 2026 there were 603 working cables in the world. Another 91 were planned or being built. They come ashore at 1,893 points, and 26 of those points are in India.

In where your question actually goes you followed one question from your hand to a machine far away. This lesson is about the part of that road nobody can see.

What is inside one of these cables

Inside the cable are long threads of glass. Each thread is called a fibre. The threads are bundled together, and that bundle is no wider than a garden hose.

Around the bundle go layers of protection. They keep out sea water, and they take the pressure of deep water. Along most of its length, the whole cable is no thicker than your arm.

That is worth pausing on. A large share of the traffic between two continents passes through something you could hold in one hand.

The glass carries light, not electricity. At one end a machine switches the light on and off, far too fast for any eye to follow. Light on is a one. Light off is a zero.

Your question leaves your phone as a long line of ones and zeros. On the cable that line becomes a long line of flashes. At the far end another machine reads the flashes and turns them back into your words.

Light in glass travels about 200,000 kilometres in one second. That is fast, but it is not instant. A data centre on another continent is thousands of kilometres away, and the light has to make the trip both ways. Part of your wait is simply the journey.

Try this in your AI app and see how it explains the same idea:

Explain how an undersea internet cable carries data.
Answer in five short points, in simple English.
Then say one thing about it that most people get wrong.

A good answer talks about glass, light and flashes. If it says radio signals travel through the water, it has gone wrong, and you now know enough to spot that.

Why Mumbai and Chennai keep coming up

A cable does not stop at the beach. It ends at a building called a landing station. Inside, the light coming in from the sea is joined to the fibre network on land.

India’s landing stations sit mostly around two cities: Mumbai and Chennai. There are others. Tata Communications runs stations at Mumbai, Chennai and Cochin. BSNL has one at Tuticorin. But the bulk of the traffic arrives at Mumbai and Chennai.

Now join that to a second fact. Data centres in India are built in the same two places. In 2025 the Mumbai region held close to half of the country’s live data centre capacity. Chennai came next.

That is not a coincidence, and it is the real insight of this lesson. A building full of computers is only useful if data can get in and out quickly. The shortest path to the rest of the world starts where the cable comes ashore. So land, power and cable meet in one spot, and the buildings go up there. Inside a data centre shows what those buildings hold.

What breaks a cable, and why you feel it here

The cables lie on the sea floor, so things happen to them.

One cause is a ship’s anchor dragging along the bottom. Fishing gear is another. So are undersea landslides, where mud and rock slide down a slope and carry the cable away.

The repair is not quick. First someone has to work out where the break is. Then a repair ship has to sail to that spot. The crew lifts the two broken ends off the sea floor, joins them together, and lowers the cable back down. Joining the glass ends is called a splice.

These are special ships, not ordinary boats, so one has to be free and has to reach the spot. Cuts in the Red Sea in February 2024 took months to repair.

A break that far away still reaches your phone. Data has more than one road. When a cable goes down, traffic shifts onto the cables that still work. That sounds like a clean fix. It is not.

The other roads are longer, so every request takes more time. Those cables also now carry their own load plus the load from the broken one. Speeds drop for everyone using that route, including you.

Picture the main road to your block being closed. Everybody moves to the narrow side road. Nobody has stopped, but everybody is late.

You can ask your AI app to walk you through this, and it usually explains it well:

An undersea internet cable is cut near another country.
I live in a village in Uttar Pradesh.
Explain in simple steps why my internet may get slower.
Keep it under 150 words.

This is a useful thing to hold in your head next time nothing loads. When the app does not work covers the faults you can actually fix. A cut cable is not one of them, and knowing that saves you an hour of restarting your phone.

Why satellites have not taken over

Satellites do carry internet, and they matter. They serve a ship in the middle of the sea. They serve a remote hill area where no cable will ever be laid.

But two things decide the main question. A single cable carries a huge amount of data at once. It also carries it cheaply. A satellite link carries far less, and each unit of data costs far more.

So satellite internet fills the gaps. It does not replace the cables. Wherever a cable can be laid, a cable carries far more data for the money.

Somebody owns the road your question travels on

A sea cable is not public property in the way a village road is. A group of companies pays to build one and then shares it. Such a group is called a consortium.

Indian names you already know are on these lists. One cable is called SEA-ME-WE-6. It runs 19,200 km and links more than 20 countries, from France to Singapore. Bharti Airtel is the company that brings both of its Indian ends ashore. Reliance Jio runs a station at Versova in Mumbai and another at Chennai, and is building more.

Very large technology companies are in this business too. Meta backs a cable called 2Africa, which has a branch planned to India, and another called Project Waterworth. Google has one called Blue-Raman, with the Raman half landing at Mumbai.

New cables often open later than promised. As of March 2026, several of these were still being tested at their Indian stations rather than fully open. If you see a claim that a cable is live, check the date on the page.

So the company that sold you your SIM may also own a piece of the sea road your question travels on. That is a strange thought, and it is true. Who builds AI, and who pays follows the same trail of owners further along the chain.

AI apps get some of this right and some of it wrong. They are usually sound on how a cable works. They are weaker on which Indian cable is open today, because that keeps changing. Treat any date or status it gives you as a claim to check, using the habits in the two-chat test and three other checks.

What to carry away from this

The internet is not in the air. It is mostly under the sea. It is glass and steel and salt water, owned by companies, cut by anchors, and mended by ships with a crew on board.

That one change of picture does real work for you. It stops the word cloud from sounding like weather. It explains a slow night without you blaming your own phone. And it tells you that when an AI answers you, something physical moved.

Next comes the bill for all of it. What it costs in electricity and water looks at the honest numbers, and at why careful people still disagree about them.