How a call crosses the ocean
A submarine cable, or undersea cable, is a cable laid on the seabed between coasts. Almost all the calls and data that cross an ocean travel through one. Your voice goes as pulses of light along glass fibres as thin as a hair, from a landing station on one coast to a landing station on the other, with repeaters on the seabed to keep the light strong. Today a call crosses the Atlantic in about a thirtieth of a second.
Purple: your callKey
- Purple: your call
- Pod on the cable: a repeater
- Dashed: mobile network
0 km of Dunant behind your callkm of TAT-1 behind your call
Both cables in service · the cable ship waits off New York
Press a button, or click a cable or a repeater to look inside it.Press a button, or tap a cable or a repeater to look inside it.
Switch to 1956 for the same call on TAT-1, the first telephone cable. It took the short way, Scotland to Newfoundland, on copper.
Anatomy of a submarine cable: one call from London to New York across the Atlantic seabed, as it works today and as it worked in 1956. The model is not to scale. It squeezes the ocean and stretches its depth.
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Follow one call
Press Play to watch the call, or scroll down and the drawing follows you one step at a time. Try it, under the drawing, makes your own call.Press Play to watch the call, or Next to go one step at a time. Try it, under the drawing, makes your own call.
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Step 1 of 6
Your call heads for the coast
You call a friend in New York from London. Your mobile reaches a mast, and the mast hands the call to your operator's exchange. The exchange sees a number in another country and sends the call along a cable under the streets, out to the coast.
Why not send the call by satellite?
A satellite in the usual high orbit sits 36,000 km up. Going up and back down adds about 260 milliseconds, written ms, in each direction. That is enough to make two people talk over each other. Under the sea, the same crossing takes about 33 ms.
That is why cables carry 99% of the internet traffic between continents, by the European Commission's count, and phone calls travel with it.
Calls between London and New York began in 1927, by radio. They crackled with the weather and cost $25 a minute.
Sources: ITU-T G.114: one-way transmission time (2003); European Commission: EU Action Plan on Cable Security (2025); IEEE History Center: telephone transmission
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Step 2 of 6
The landing station sends it out to sea
By the beach stands a landing station, a building with no windows where the cables come ashore. Your call goes out on Cable 1. The cable runs from the station to a manhole on the beach, and from there into the sea. The station also sends an electric current down the cable to power the repeaters on the way.
Why is the cable thicker near the shore?
Near the coast, fishing gear and ship anchors reach the seabed. So in shallow water the cable wears a coat of steel wires that brings it to 40 to 50 mm across, and on the continental shelf it is usually buried about a metre down in the seabed.
Even so, 65 to 75% of faults on fibre cables happen in water less than 200 m deep.
Sources: UNEP-WCMC and ICPC: Submarine cables and the oceans (2009)
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Step 3 of 6
The cable lies on the deep seabed
Past the continental shelf, the cable runs down the slope and lies on the bare seabed, about 4 km down, then climbs over the Mid-Atlantic Ridge. Out here it is no thicker than a garden hose. Inside, glass fibres carry your voice as pulses of light. Dunant, a cable from Virginia Beach to the French coast, is 6,600 km long.
Look inside The cable
- 1Polyethylene
- 2Copper sheath
- 3Steel wires
- 4Tube of gel
- 5Glass fibres
Almost half the world could be on the phone at once on Dunant alone, or 40 million people could watch a film in HD. It carries 250 terabits a second: 250 million million ones and zeros.
- 1Tarred jute
- 2Steel armour
- 3Jute bedding
- 4Copper tapes
- 5Polyethylene
- 6Copper core
35 calls at a time on TAT-1, plus one for telegraph. One cable today carries about 55 million times as many.
How long does the crossing take?
Light in glass fibre needs about 5 millionths of a second for each kilometre. Over 6,600 km that adds up to about 33 ms, a thirtieth of a second. The drawing runs the light 200 times slower so you can follow it.
Phone engineers aim to keep the delay from mouth to ear under 150 ms. Below that, most people never notice it.
Sources: ITU-T G.114: one-way transmission time (2003); Orange: the Dunant cable lands in France (2020); Bell System Technical Journal: TAT-1, planning and performance (1957)
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Step 4 of 6
Repeaters keep the light strong
Light fades as it travels through glass. So every 70 km or so, the cable passes through a repeater, a pod that boosts the light and sends it on. The drawing shows eight of them. A real cable across the Atlantic has dozens.
Look inside A repeater
- 1Housing
- 2Coupling
- 3Erbium fibre
- 4Pump lasers
- 5Power supply
25 years on the seabed, the life a repeater is built for
- 1Copper tube
- 2Steel rings
- 3Sections
- 4Vacuum tubes
51 repeaters on each cable, one every 69 km
Where does a repeater get its power?
From the shore. Equipment in the landing stations sends a steady current along a conductor inside the cable, and the sea carries it back. In 1956, TAT-1's stations fed 1,950 volts in from each end.
TAT-1 had 51 repeaters in each of its two cables, each with three vacuum tubes. Not one tube failed in 22 years of service.
Sources: UNEP-WCMC and ICPC: Submarine cables and the oceans (2009); ITU-T G.977: optically amplified submarine cable systems (2015); Bell System Technical Journal: TAT-1, power feed equipment (1957); IEEE: Milestone, TAT-1, the first transatlantic telephone cable (2006)
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Step 5 of 6
Your friend picks up in New York
The cable climbs the far slope, comes ashore through a manhole on an American beach and reaches the landing station there. The call runs on to the exchange, up the mast and to your friend's phone. Your voice has crossed the Atlantic in about 33 ms, and the answer comes back through the same cable.
Was it slower in 1956?
Hardly. On TAT-1, a call from London to New York took 35 ms to arrive, and an operator put it through. What changed is how many calls fit. TAT-1 had room for 35 calls at a time, one cable for each direction. TAT-8, the first fibre cable across the Atlantic, carried 40,000 in 1988. Dunant carries 250 terabits per second, which Google says is enough to send the whole digitised Library of Congress three times a second.
What is a terabit? A bit is a one or a zero. In the fibre, a flash of light is a one and no flash is a zero, and the laser flashes extremely fast. Each fibre also carries many colours of light side by side, each with its own stream of flashes. A terabit is a million million bits.
For phone calls, that is easier to picture. A call needs 64 kilobits per second in each direction. So Dunant alone has room for about 2 billion calls at once: almost half the 8.2 billion people on Earth could be on the phone together, through one cable. Or, since Netflix asks for 3 megabits per second for HD, about 40 million people could watch a film in HD at the same time.
Sources: Bell System Technical Journal: TAT-1, planning and performance (1957); IEEE: Milestone, TAT-8, the first fibre-optic cable across an ocean (2016); Google: Dunant is ready for service (2021); ITU-T G.711: pulse code modulation of voice frequencies (1988); United Nations: World Population Prospects 2024, summary of results (2024); Nobel Prize: popular information, the physics prize (2009); ITU-T G.694.1: spectral grids for WDM applications (2020); Netflix: internet connection speed recommendations
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Step 6 of 6
When a cable breaks, calls take another
A fishing trawler drags its net across Cable 1 near the coast and breaks it. Fishing gear and ship anchors cause most breaks. The network moves traffic onto other cables by itself, so your call takes Cable 2 and you never notice. A cable ship then sails out, hooks the cable with a grapnel and lifts both ends aboard. On deck, the crew splices in a new piece and lays the cable back on the seabed.
How long does a repair take?
Every year brings 150 to 200 reported faults on submarine cables around the world, about three a week.
Weeks, often. About 65 cable ships work around the world, and only about 20 of them mainly do repairs. In 2024 a repair took 51 days on average.
Once the ends are on deck, joining the hair-thin fibres again and testing them takes 10 to 24 hours.
Sources: UK Parliament: Subsea telecommunications cables, resilience and crisis preparedness (2025); US Office of Naval Intelligence: answer to the US-China Commission on cable repair (2026); ITU: summit on the resilience of submarine cables (2025); UNEP-WCMC and ICPC: Submarine cables and the oceans (2009)
Look inside: 1956 and today
Your voice crosses only about 2 ms faster than in 1956. What changed is the room: one cable now carries about 55 million times as many calls.
The same cable and the same repeater, 70 years apart. Point at a part, or tap it, to see what it does.
TAT-1, 1956
Dunant, today
The cable
- 1Tarred jute
- 2Steel armour
- 3Jute bedding
- 4Copper tapes
- 5Polyethylene
- 6Copper core
35 calls at a time on TAT-1, plus one for telegraph. One cable today carries about 55 million times as many.
The cable
- 1Polyethylene
- 2Copper sheath
- 3Steel wires
- 4Tube of gel
- 5Glass fibres
Almost half the world could be on the phone at once on Dunant alone, or 40 million people could watch a film in HD. It carries 250 terabits a second: 250 million million ones and zeros.
Calls at onceBell System Technical Journal; Google; ITU-T G.711
35
About 2 billion
What carries your voiceIEEE; Google
Copper
Electric current in a copper core
Light
Pulses of light in 12 pairs of glass fibres
London to New YorkBell System Technical Journal; ITU-T G.114
35 ms
About 33 ms
Boosters on the wayIEEE; ITU-T G.977
51 repeaters
On each cable, with three vacuum tubes in each
Every 70 km
A repeater, with lasers instead of tubes
How long it lastsIEEE; International Cable Protection Committee
22 years
From 1956 to 1978
25 years
What a cable is built to last
A repeater
- 1Copper tube
- 2Steel rings
- 3Sections
- 4Vacuum tubes
51 repeaters on each cable, one every 69 km
A repeater
- 1Housing
- 2Coupling
- 3Erbium fibre
- 4Pump lasers
- 5Power supply
25 years on the seabed, the life a repeater is built for
How it came to be
- 1858
A greeting that took hours
The first telegraph cable across the Atlantic carries Queen Victoria's greeting to the President of the United States. Sending it takes hours, and the cable stops working within weeks.
Library of Congress: the first transatlantic telegraph cable (1858)
- 1866
The cable that lasted
The Great Eastern, by far the largest ship afloat, lays a telegraph cable from Ireland to Newfoundland in 14 days. This one works, and the two continents stay in touch.
- 1927
Calls go by radio
Telephone service opens between Britain and the United States over a radio link. The line crackles with the weather, and a call costs $25 a minute.
- 1956
The first telephone cable
TAT-1 opens on 25 September, with two cables between Scotland and Newfoundland. On its first day it carries 707 calls between London and North America.
- 1988
Light instead of electricity
TAT-8, the first fibre-optic cable to cross an ocean, enters service on 14 December. It can carry 40,000 calls at once, ten times the last copper cable.
IEEE: Milestone, TAT-8, the first fibre-optic cable across an ocean (2016)
- 2025
Guarding the cables
After cables in the Baltic Sea are damaged, the European Union publishes an action plan to protect submarine cables and to keep repair ships ready for them.
European Commission: EU Action Plan on Cable Security (2025)
Who pays for the cables
The cable your call just crossed belongs to Google. SubCom built it, and Orange owns the French end. In 1956, TAT-1 belonged to three phone companies, and for decades phone companies paid for cables that way, sharing the bill. Now internet companies pay for many new ones themselves.
What one cable costs, and who built it
Each price is the one the project published, in its own currency. Governments and the European Union's bank paid part of the first two. For scale, Alcatel Submarine Networks sells about €1 billion a year across all its projects, roughly three Medusas.
European Investment Bank; US Department of State; Google; Alcatel Submarine Networks
Who owns one cable
17 owners
The Europe India Gateway cable has 17 owners, all phone companies. For most of the cables' history, the business ran like this, as a club of carriers.
Cables into one country
About 64
That's how many cables land in the United Kingdom, where your call starts. The darker dots are the 45 that run to other countries. About 50 are thought to be in use.
The old France Télécom
More than 40
That's how many submarine cables Orange, called France Télécom until 2013, has invested in. On Dunant it owns the French end, runs the landing station and gets two of the 12 fibre pairs.
Who pays now
- 2010 Unity enters service, the first cable Google invested in.
- 2017 Internet companies such as Google, Meta, Microsoft and Amazon now use more ocean cable capacity than the carriers do.
- 2026 to 2029 US$16 billion of new cables are due to enter service. Spending like this has rarely been seen since 2000 and 2001. Over the last nine years it averaged more than US$2 billion a year.
Where Voxbi fits
Voxbi, a cloud phone system hosted in the European Union, sends your calls abroad over these cables. Its call rates page lists the price per minute for each country.
See it in Voxbi