# How Undersea Cables Work > Meta has announced Petal, a cable designed to carry a petabit per second between the United States and France from 2029. This film explains how undersea cables work: from the telegraph cable of 1858 to the first fibre-optic cable across the Atlantic in 1988, how glass traps light by total internal reflection, how fast it travels, what a cable is made of, how repeaters powered from shore keep the light strong, how many colours and now several cores share one fibre, and why about 150 to 200 cables are damaged every year. Bend the light, add colours, switch off the amplifiers and drag an anchor. Interactive film: https://www.planetpast.com/stories/how-undersea-cables-work/ Publisher: PlanetPast (https://www.planetpast.com/) · d => T('readPublished', l, { d: dateIn(d, l) }) 2026-10-19 · Technology, Physics, History ## Prologue ### Light on the ocean floor *A cable in the news* A petabit a second, under the Atlantic. Almost all the data that travels between continents goes along the ocean floor, as light inside glass. This autumn, Meta announced the newest of these cables, called Petal. It will run about seven thousand kilometres between the United States and France from 2029, and is designed to carry one petabit per second: a million billion bits, every second. A bit is a single one or zero, the smallest piece of digital data. How do you send that many of them across an ocean, through glass? - **≈ 7,000 km**: United States to France - **2029**: expected in service $$1\ \text{Pbps} = 10^{15}\ \text{bits per second}$$ About double today’s most advanced cables over that distance, says Meta. ![Satellites carry just 0.37 percent of US international capacity, according to FCC data cited by TeleGeography. AI-generated still.](https://assets.planetpast.com/a/8a/8a42bca2c56cf9a7.jpg) Prologue · The news #### Petal Meta, 21 September 2026: Petal will connect France and the United States over about 7,000 km with a design capacity of 1 Pbps (1,000 Tbps), about double today’s most advanced subsea cables at that distance, and is expected in service in 2029. NEC builds it; Sumitomo Electric supplies the fibre; Orange lands it in France. Live Science reported it on 17 October 2026. These are design figures for a cable not yet built. How to use: pick a chapter or press ← →; drag to rotate. Most steps have buttons. The 3D scenes are schematic: distances, depths and sizes are not to scale. Sources: [Meta Engineering: Petal, a petabit transoceanic subsea cable (21 Sept 2026)](https://engineering.fb.com/2026/09/21/connectivity/petal-petabit-transoceanic-subsea-cable/); [Live Science: Meta’s 4,300-mile-long internet cable will shuttle data at 1 million gigabits per second (17 Oct 2026)](https://www.livescience.com/technology/communications/new-4-300-mile-long-internet-cable-will-shuttle-data-along-the-ocean-floor-at-1-million-gigabits-per-second); [TeleGeography: Submarine cable frequently asked questions (2026)](https://www2.telegeography.com/submarine-cable-faqs-frequently-asked-questions); [International Cable Protection Committee: media enquiries and facts](https://iscpc.org/news/media-enquiries/) ## Chapter I · Wires under the sea ### Ninety-eight words *Chapter I · Wires under the sea* The first message across the Atlantic. The idea is old. In 1858 a team financed by the American businessman Cyrus Field laid the first telegraph cable across the Atlantic: copper wire, insulated and laid on the sea bed. On the sixteenth of August, Queen Victoria sent President James Buchanan a message of ninety-eight words. It took sixteen and a half hours to get through. Within weeks, the cable failed. In 1866 the giant steamship Great Eastern laid a tougher cable that worked, and sent messages about eighty times faster. ![Telegraph signals were pulses of electric current in copper, read as dots and dashes. AI-generated still.](https://assets.planetpast.com/a/83/83ece5e8bffafbfc.jpg) - **98**: words - **16.5 hours**: to transmit - 1858: first cable; fails within weeks - 1866: Great Eastern lays a lasting cable - 1988: TAT-8, the first fibre-optic cable - 2029: Petal, planned Chapter I · Wires under the sea #### The telegraph cables The Henry Ford: the first transatlantic cable was laid in 1858; on 16 August Queen Victoria sent a 98-word message to President James Buchanan that took 16.5 hours; the cable faltered and failed about three weeks after it was laid. In 1866 the Great Eastern completed the first successful cable, more durable and able to send messages about eighty times faster. Cyrus Field financed both. Sources: [The Henry Ford: Starts and stalls, creating a successful transatlantic cable](https://www.thehenryford.org/collections-and-research/digital-collections/expert-sets/101439/) ### The first glass cable *Chapter I · Wires under the sea* TAT-8, 1988. For more than a century, ocean cables carried electricity through copper: first telegraph pulses, later telephone calls. Then came glass. On the fourteenth of December 1988, a cable called TAT-8 began carrying light across the Atlantic in optical fibres. It could carry about forty thousand telephone calls at once, roughly ten times the last copper cable. Since then, the fastest cables have become millions of times faster. In the chart, each step up the scale means ten times more. - **copper**: electric signals, from 1858 - **glass**: light, from 1988 ![An IEEE Milestone: the first fibre-optic cable to cross an ocean, 5,846 km of seabed, led by AT&T, British Telecom and France Telecom. AI-generated still.](https://assets.planetpast.com/a/12/1276275d46c49a46.jpg) - TAT-8, 1988, per repeater: 280 Mbps - 1990s, 2nd generation: 560 Mbps - TAT-12/13, 1996: 5 Gbps - Petal, planned: 1 Pbps Chapter I · Wires under the sea #### TAT-8 IEEE Milestone: TAT-8 entered service on 14 December 1988, crossing 5,846 km of seabed between North America and Europe. It carried about 40,000 telephone circuits, about ten times the last copper transatlantic cable, at 280 Mbps per repeater over single-mode fibre at 1.3 microns. Laser Focus World (1997): second-generation systems ran at 560 Mbps; TAT-12/13, in service in 1996, was designed for 5 Gbps. The 3D bars use a logarithmic scale; the figures are not like-for-like (per repeater, per system, per cable) and show the trend only. Sources: [IEEE Milestone: Trans-Atlantic Telephone Fiber-Optic Submarine Cable (TAT-8), 1988](https://ethw.org/Milestones:Trans-Atlantic_Telephone_Fiber-Optic_Submarine_Cable_%28TAT-8%29%2C_1988); [Laser Focus World: Optical amplifiers speed data flow undersea (1997)](https://www.laserfocusworld.com/fiber-optics/article/16551090/optical-amplifiers-speed-data-flow-undersea); [Wikipedia: TAT-12/13 (in service 1996)](https://en.wikipedia.org/wiki/TAT-12/13); [Meta Engineering: Petal, a petabit transoceanic subsea cable (21 Sept 2026)](https://engineering.fb.com/2026/09/21/connectivity/petal-petabit-transoceanic-subsea-cable/) ## Chapter II · Light in glass ### Data as light *Chapter II · Light in glass* A laser at one end, a detector at the other. Inside the cable, data travels as light. At the sending end, a laser turns electrical signals into pulses of light, and the pattern of pulses spells out the ones and zeros of digital data. At the far end, a light detector called a photodetector turns each pulse back into an electrical pulse that computers can read. Digital pulses survive long distances well, because a little noise does not turn a one into a zero. Above about twenty-five billion bits a second, simply flashing a laser on and off reaches its limit, so the fastest links use more elaborate methods, called coherent systems. - transmitter: electrical signal → light (laser) - fibre: carries the light, losing a little on the way - receiver: light → electrical pulse (photodetector) - **1**: a flash of light - **0**: no flash > Digital signals can be transmitted long distances without degradation as the signal is less sensitive to noise. (Fiber Optic Association) Chapter II · Light in glass #### The datalink Fiber Optic Association: the transmitter takes an electrical input and converts it to an optical output from a laser diode or LED; at the receiver the light pulse is converted to an electrical pulse by a photodetector. Most links carry digital data, which can travel long distances without degradation. Above about 25 Gb/s, direct modulation of typical lasers reaches its practical limit, and higher-rate links use coherent systems. The pulses in the scene are schematic. Sources: [Fiber Optic Association: The fiber optic datalink](https://thefoa.org/tech/ref/OSP/OSPdatalink.html) ### A mirror made of glass *Chapter II · Light in glass* How a fibre keeps light inside. An optical fibre is a strand of very pure glass, about as thick as a human hair. Its centre, the core, is wrapped in a second layer of glass called the cladding. The core slows light slightly more than the cladding does. Because of that difference, light that meets the boundary at a shallow, grazing angle, almost parallel to the fibre, is reflected back instead of passing through. It bounces again and again, all the way down the fibre. This is called total internal reflection. Now watch the angle get steeper: past a limit, the light leaks out. ![TeleGeography: individual fibres are roughly the diameter of a human hair. AI-generated still.](https://assets.planetpast.com/a/ec/ec365e522e337631.jpg) - **≈ 1.46**: core - **≈ 1.45**: cladding $$n = \dfrac{c}{v}$$ - $n$: refractive index - $c$: speed of light in vacuum - $v$: speed of light in the glass A higher index means slower light; the small difference is enough to trap it. Chapter II · Light in glass #### Total internal reflection Fiber Optic Association: the core glass has a slightly higher refractive index (about 1.46) than the cladding (about 1.45). Light meeting the core–cladding boundary at a shallow angle is reflected back into the core; light at steeper angles is refracted into the cladding and lost. With these indices the critical angle is about 83° from the normal, so rays within about 7° of the fibre axis stay trapped. In the scene the fibre is drawn hugely enlarged and the angles exaggerated. Sources: [Fiber Optic Association: Total internal reflection](https://www.thefoa.org/tech/ref/basic/total_internal_reflection.html); [TeleGeography: Submarine cable frequently asked questions (2026)](https://www2.telegeography.com/submarine-cable-faqs-frequently-asked-questions) ### How fast is fast? *Chapter II · Light in glass* Light in glass, across an ocean. Light in glass is slower than light in empty space. Divide the speed of light by the refractive index, about one point four six, and you get roughly two hundred and five thousand kilometres per second. That is still fast. Across Petal's seven thousand kilometres, a pulse of light would take about thirty-four thousandths of a second. That delay is called latency, and for video calls and online games it matters. Watch two pulses race: one through empty space, one through glass. $$v = \dfrac{c}{n} \approx \dfrac{299{,}792\ \text{km/s}}{1.46} \approx 205{,}000\ \text{km/s}$$ About two-thirds of the speed of light in a vacuum. $$t = \dfrac{7{,}000\ \text{km}}{205{,}000\ \text{km/s}} \approx 0.034\ \text{s}$$ One way. A reply doubles it, before any equipment delays. - **≈ 34 ms**: light in glass (computed) - **16.5 h**: the 1858 message, for comparison Chapter II · Light in glass #### Latency From n = c/v with n ≈ 1.46 (Fiber Optic Association), light in the core travels at about 299,792 / 1.46 ≈ 205,000 km/s. Over Petal’s planned 7,000 km that is about 34 milliseconds one way. Real systems add delays in equipment, and routes are rarely straight. The latency figure is computed, not measured. Sources: [Fiber Optic Association: Total internal reflection](https://www.thefoa.org/tech/ref/basic/total_internal_reflection.html); [Meta Engineering: Petal, a petabit transoceanic subsea cable (21 Sept 2026)](https://engineering.fb.com/2026/09/21/connectivity/petal-petabit-transoceanic-subsea-cable/); [The Henry Ford: Starts and stalls, creating a successful transatlantic cable](https://www.thehenryford.org/collections-and-research/digital-collections/expert-sets/101439/) ## Chapter III · The cable ### As thick as a garden hose *Chapter III · The cable* A few fibres, a lot of protection. For most of its length, an ocean cable is only about as thick as a garden hose. The glass fibres sit at the very centre, inside layers that protect them and carry electrical power. In the deep ocean, where few ships reach the bottom, the cable simply rests on the sea floor with a few layers of insulation. Near the shore it gets heavy steel armour and is buried under the seabed. Inside, the fibres work in pairs, one for each direction. Cables are designed to last at least twenty-five years. - optical fibres - strength members - a metal conductor carrying power - insulation and water-blocking materials - near shore: steel-wire armour ![Shore-side sections are buried under the seabed; deep-sea sections rest on the ocean floor (TeleGeography). AI-generated still.](https://assets.planetpast.com/a/07/0706138767888bc8.jpg) - **25 years**: minimum design life - **often less**: retired when out of date Chapter III · The cable #### What a cable is made of TeleGeography: most of a cable’s length is about as wide as a garden hose; individual fibres are about the diameter of a hair. Cables near shore have extra armour and are buried; deep-sea cables rest on the ocean floor with a few layers of insulation and protection. Minimum design life is 25 years, though cables are often retired earlier when they become economically obsolete. SubTel Forum: a cable’s general layers are optical fibres, strength members, insulation, a metallic power conductor and water-blocking materials; near shore, steel wires around the core add armour. A fibre pair normally uses one fibre for each direction. The cut-away in the scene is schematic. Sources: [TeleGeography: Submarine cable frequently asked questions (2026)](https://www2.telegeography.com/submarine-cable-faqs-frequently-asked-questions); [SubTel Forum: How submarine fiber-optic cables work](https://subtelforum.com/how-submarine-cables-work/) ### Topping up the light *Chapter III · The cable* Amplifiers on the sea floor. Even the purest glass absorbs a little light. After tens of kilometres, the signal grows faint. So along the cable sit repeaters: sealed boxes that boost the light, about every seventy kilometres. Since the mid-nineteen-nineties they have used a stretch of fibre laced with the element erbium. A pump laser feeds energy into the erbium, which then amplifies the passing light directly, without turning it into electricity. A cable as long as Petal typically needs about a hundred of them, powered through the cable from the shore at up to eighteen thousand volts. Watch what happens without them. ![Laser Focus World (1997): noise and other effects limited sections to roughly 75–90 km. AI-generated still.](https://assets.planetpast.com/a/a8/a8a4d3a5ca252044.jpg) - pumped by a laser at 980 or 1480 nm - amplifies light around 1550 nm - about 12–16 dB of gain each - **≈ 100**: repeaters on 7,000 km - **18 kV**: power-feed rating Chapter III · The cable #### Repeaters Laser Focus World (1997): erbium-doped fibre amplifiers, pumped at 980 or 1480 nm and emitting at 1550 nm, replaced electronic regenerators, removing high-speed electronics from repeaters. First-generation systems put repeaters about 70 km apart; noise limits sections to roughly 75–90 km. Meta: a 7,000 km cable typically needs about 100 repeaters; Petal stays within power-feed equipment rated up to 18 kV. The signal-strength curve in the scene is schematic. Sources: [Laser Focus World: Optical amplifiers speed data flow undersea (1997)](https://www.laserfocusworld.com/fiber-optics/article/16551090/optical-amplifiers-speed-data-flow-undersea); [Wikipedia: TAT-12/13 (in service 1996)](https://en.wikipedia.org/wiki/TAT-12/13); [Meta Engineering: Petal, a petabit transoceanic subsea cable (21 Sept 2026)](https://engineering.fb.com/2026/09/21/connectivity/petal-petabit-transoceanic-subsea-cable/) ## Chapter IV · More in one fibre ### A rainbow in a hair *Chapter IV · More in one fibre* Each colour carries its own data. One colour of light in one fibre can carry a lot. But light of slightly different colours, different wavelengths, can share the same fibre without mixing. This is called wavelength-division multiplexing. Each colour carries its own stream of data. Typical systems use forty or eighty colours, and some use up to a hundred and sixty. At the far end, the colours are separated again. Watch the capacity climb as colours are added. - “multiplex”: many signals sharing one path - here, each signal gets its own wavelength - **40–80**: channels in typical systems - **160**: channels at most, typically - 50 GHz: 0.4 nm - 100 GHz: 0.8 nm - 200 GHz: 1.6 nm Chapter IV · More in one fibre #### Wavelength-division multiplexing EXFO: dense wavelength-division multiplexing increases a fibre’s bandwidth by sending several wavelengths over it at once. Channel spacing is 50, 100 or 200 GHz (0.4, 0.8 or 1.6 nm); typical systems use 40 or 80 channels and can reach 160. The C-band is about 1525–1565 nm, where erbium-doped amplifiers work. The “colours” are infrared; the scene shows them as visible colours for clarity. Sources: [EXFO: Dense wavelength-division multiplexing (DWDM)](https://www.exfo.com/es/recursos/glossary/dwdm/) ### More cores *Chapter IV · More in one fibre* Petal’s trick. Engineers can also add more fibres. Petal does something new: each fibre has two cores instead of one, inside the same hair-thin glass. With twenty-four fibre pairs, each holding two cores, it behaves like forty-eight pairs. Meta says this is the first ocean cable to use multi-core fibre at scale. The hard part is keeping the light in the two cores from leaking into each other. ![Same 125-micrometre outer diameter as ordinary fibre, made by Sumitomo Electric. AI-generated still.](https://assets.planetpast.com/a/6c/6cdc000a0e071564.jpg) - MAREA: 8 fibre pairs: 8 - Amitié: 16 pairs: 16 - Anjana: 24 pairs, 0.5 Pbps: 24 - Petal: 24 two-core pairs: ≈ 48 - careful control of each core’s refractive index - signals in neighbouring cores sent in opposite directions - ultra-pure synthetic silica to keep losses low Chapter IV · More in one fibre #### Multi-core fibre Meta: Petal uses 2-core fibre in a 24 fibre-pair system, equivalent to 48 pairs, the first subsea cable to deploy multi-core fibre at scale. The fibre keeps the standard 125 µm outer diameter using ultra-pure synthetic silica, and limits crosstalk by controlling refractive indices and counter-propagating signals. Meta compares it with MAREA (8 pairs), Amitié (16) and Anjana (24 pairs, 0.5 Pbps); Petal’s capacity is 5.5 times MAREA’s. Sources: [Meta Engineering: Petal, a petabit transoceanic subsea cable (21 Sept 2026)](https://engineering.fb.com/2026/09/21/connectivity/petal-petabit-transoceanic-subsea-cable/); [Live Science: Meta’s 4,300-mile-long internet cable will shuttle data at 1 million gigabits per second (17 Oct 2026)](https://www.livescience.com/technology/communications/new-4-300-mile-long-internet-cable-will-shuttle-data-along-the-ocean-floor-at-1-million-gigabits-per-second); [DatacenterDynamics: Meta plans Petal, the first transoceanic subsea cable at petabit capacity](https://datacenterdynamics.com/en/news/meta-plans-petal-the-first-transoceanic-subsea-cable-at-petabit-capacity) ## Chapter V · When cables break ### Anchors and nets *Chapter V · When cables break* Most damage is an accident. Cables break more often than you might think. The International Cable Protection Committee counts about one hundred and fifty to two hundred faults a year worldwide. Most are accidents: fishing gear dragged along the bottom, or a ship's anchor. Underwater landslides, currents and storms cause some. Shark bites, despite the stories, are extremely rare. Watch an anchor drag across this one. When a cable breaks, traffic shifts to other cables while a repair ship comes to fix the damaged section. - **≈ 1.7 M km**: of cable - **≈ 500**: cable systems ![ICPC: 70–80 percent of faults come from accidental human activity, mainly fishing and anchors. AI-generated still.](https://assets.planetpast.com/a/5b/5b3acffbfa5f2360.jpg) - **0**: faults from fish bites, 2007–2014 (TeleGeography) - **rare**: sabotage, also Chapter V · When cables break #### Cable faults ICPC: about 150–200 submarine cable faults occur worldwide each year, a number roughly stable as total length has grown; about 70–80% come from accidental human activity, mainly fishing and anchors, with others from abrasion, equipment failure and natural hazards. TeleGeography: fishing vessels and anchors cause about two-thirds of faults; sabotage and shark bites are exceedingly rare; fish bites caused no faults from 2007 to 2014. Networks keep running on other cables while one is repaired. The two sources give slightly different shares (two-thirds; 70–80%). The repair scene is schematic. Sources: [International Cable Protection Committee: media enquiries and facts](https://iscpc.org/news/media-enquiries/); [TeleGeography: Submarine cable frequently asked questions (2026)](https://www2.telegeography.com/submarine-cable-faqs-frequently-asked-questions) ### Six hundred cables *Chapter V · When cables break* Who builds them, and why there are so many. Today more than six hundred cables are active or planned, with well over one and a half million kilometres of cable lying in the oceans. For a long time they were built by groups of telephone companies. Now companies such as Google, Meta, Microsoft and Amazon are among the biggest investors. Countries are safest with several cables, so that one break does not cut them off. That is why the web keeps growing. - **600+**: cables active or planned - **1.5 M km+**: in service - CeltixConnect, Ireland–UK: 131 km - Petal, US–France (planned): ≈ 7,000 km - Asia America Gateway: 20,000 km - New York / New Jersey - Porthcurno, England - Bude, England Chapter V · When cables break #### The global network TeleGeography (2026): more than 600 active and planned cables; over 1.5 million km in service (ICPC gives about 1.7–1.8 million km across roughly 500 systems). Carriers traditionally owned cables through consortia; content providers such as Google, Meta, Microsoft and Amazon are now major investors. Countries should have multiple cables for reliability. Counts differ between trackers and over time; the network in the scene is schematic. Sources: [TeleGeography: Submarine cable frequently asked questions (2026)](https://www2.telegeography.com/submarine-cable-faqs-frequently-asked-questions); [International Cable Protection Committee: media enquiries and facts](https://iscpc.org/news/media-enquiries/) ## Epilogue ### Flashes in the dark *Epilogue* The internet’s hidden backbone. So the next time a page loads from another continent, picture it: flashes of infrared light, trapped in glass by total internal reflection, many colours at once. Boosted every seventy kilometres or so by erbium, crossing the dark sea floor in a few hundredths of a second. From ninety-eight words in sixteen and a half hours in 1858, to a million billion bits every second. Light in glass cannot go any faster, so the next gains will come from more colours, more cores and more cables. - 1858: first transatlantic telegraph cable - 1866: Great Eastern’s lasting cable - 1988: TAT-8: first fibre-optic ocean cable - 1990s: erbium amplifiers replace regenerators (TAT-12/13, 1996) - 2029: Petal: multi-core, 1 Pbps (planned) - glass traps light by total internal reflection - repeaters top up the light along the way - many colours and cores share one cable - **98 words / 16.5 h**: 1858 - **10¹⁵ bits / s**: Petal, planned Epilogue #### Sources Every figure in this film comes from the sources below. Petal figures are design figures for a cable still to be built. Stills are AI-generated illustrations, not photographs. The 3D scenes are schematic: distances, depths and sizes are not to scale. Sources: [Meta Engineering: Petal, a petabit transoceanic subsea cable (21 Sept 2026)](https://engineering.fb.com/2026/09/21/connectivity/petal-petabit-transoceanic-subsea-cable/); [Live Science: Meta’s 4,300-mile-long internet cable will shuttle data at 1 million gigabits per second (17 Oct 2026)](https://www.livescience.com/technology/communications/new-4-300-mile-long-internet-cable-will-shuttle-data-along-the-ocean-floor-at-1-million-gigabits-per-second); [DatacenterDynamics: Meta plans Petal, the first transoceanic subsea cable at petabit capacity](https://datacenterdynamics.com/en/news/meta-plans-petal-the-first-transoceanic-subsea-cable-at-petabit-capacity); [TeleGeography: Submarine cable frequently asked questions (2026)](https://www2.telegeography.com/submarine-cable-faqs-frequently-asked-questions); [International Cable Protection Committee: media enquiries and facts](https://iscpc.org/news/media-enquiries/); [SubTel Forum: How submarine fiber-optic cables work](https://subtelforum.com/how-submarine-cables-work/); [The Henry Ford: Starts and stalls, creating a successful transatlantic cable](https://www.thehenryford.org/collections-and-research/digital-collections/expert-sets/101439/); [IEEE Milestone: Trans-Atlantic Telephone Fiber-Optic Submarine Cable (TAT-8), 1988](https://ethw.org/Milestones:Trans-Atlantic_Telephone_Fiber-Optic_Submarine_Cable_%28TAT-8%29%2C_1988); [Wikipedia: TAT-12/13 (in service 1996)](https://en.wikipedia.org/wiki/TAT-12/13); [Laser Focus World: Optical amplifiers speed data flow undersea (1997)](https://www.laserfocusworld.com/fiber-optics/article/16551090/optical-amplifiers-speed-data-flow-undersea); [Fiber Optic Association: The fiber optic datalink](https://thefoa.org/tech/ref/OSP/OSPdatalink.html); [Fiber Optic Association: Total internal reflection](https://www.thefoa.org/tech/ref/basic/total_internal_reflection.html); [EXFO: Dense wavelength-division multiplexing (DWDM)](https://www.exfo.com/es/recursos/glossary/dwdm/)