Undersea cables Vietnam depends on are the thin lifelines of its internet

Undersea cables Vietnam depends on are thin glass threads lying on the ocean floor, each no thicker than a garden hose, carrying almost everything the country sends to and receives from the rest of the internet. When you watch a video hosted abroad or back up files to a server overseas, your data almost certainly travels through one of these cables. Vietnam connects to the world through six main submarine cable systems — AAG, IA, AAE-1, APG, ADC and SJC2 — and when several fail at once, the whole country feels it: videos buffer, games lag and overseas websites load slowly. The cables are invisible, but they are a fragile part of Vietnam’s digital infrastructure.

The internet arrives by sea

It surprises many people that the internet travels by sea. Satellites carry only a tiny fraction of global data; more than 99 percent of intercontinental internet traffic moves through fibre-optic cables laid on the seabed. A cable is a bundle of hair-thin glass fibres wrapped in steel and plastic, carrying pulses of light that encode data. The newest systems can move well over 100 terabits per second.

Vietnam’s cables come ashore at landing stations on the central and southern coast, including Vung Tau, Quy Nhon and Da Nang, and from there the traffic fans out across the country on domestic fibre. The landing station is a small, heavily secured building where the undersea cable meets the land network; without it, the cable is useless. Because Vietnam’s long coastline faces the sea routes between East Asia and Southeast Asia, the country is well placed to host landing stations — and exposed when the cables that serve it run into trouble.

The six cables that carry a country

Each of Vietnam’s six main cables has its own history, route and owners. The Asia-America Gateway (AAG) is the oldest, connecting Southeast Asia with the United States across the Pacific. The Intra Asia cable (IA) links the region’s hubs. AAE-1 runs from Asia through the Middle East and Africa to Europe, and Viettel manages its landing station in Vietnam after investing 50 million US dollars. The Asia Pacific Gateway (APG) connects the region’s major economies. The Asia Direct Cable (ADC) is one of the newest, with a maximum design capacity of 50 terabits per second, only partly activated so far. And the Southeast Asia–Japan Cable 2 (SJC2), the newest of all, has a design capacity of 126 terabits per second, with VNPT as an investor and a landing station in Quy Nhon.

The total capacity sounds enormous, and it is — but capacity on paper is not the same as capacity in use. A system that is only partly activated carries only a fraction of its design load, so when one or two cables go down, the rest have to absorb traffic they were not sized for.

When the cables fail

Failures are not rare. They have become a recurring feature of Vietnamese internet life, happening several times a year. In 2023, all five cables then in service were down simultaneously at one point. In 2024, three cables — IA, APG and AAE-1 — were hit by outages. In late August 2026, four cables developed faults almost at once: SJC2 failed in its S4 segment near Chung Hom Kok in Hong Kong, AAE-1 lost connectivity in its S1.H2 segment, and the AAG and IA cables were also affected. Operators estimated the incident cut about 30 percent of Vietnam’s international internet traffic.

The experience for users is distinctive. Websites and services hosted inside Vietnam keep working normally, because domestic traffic never touches the submarine cables. What slows down is everything hosted abroad: video platforms, social media, cloud services, games on foreign servers. The slowdown is worst in the evening, when millions of people come online at once. Providers respond by balancing loads, rerouting traffic and buying extra capacity on the cables that still work — Viettel added 800 gigabits per second through the IA cable and 300 gigabits per second through APG during the August 2026 incident — but there is only so much spare capacity to buy.

Why repairs take so long

Fixing a submarine cable is nothing like fixing a road. First, the fault has to be located by sending light pulses down the fibre and measuring the reflections, which can narrow the break to a stretch of a few kilometres. Then a specialised repair ship must sail to the location, which can take days if the ship is working elsewhere. The crew hauls the cable up from the seabed, sometimes from depths of thousands of metres, cuts out the damaged section, splices in a replacement and lowers it back down.

Weather, permits and ship availability all add delay, and the work can only be done in suitable sea conditions. In the August 2026 incident, repairs on the AAG and IA cables were expected to begin in September, weeks after the faults appeared, while no timetable was announced for the others. The AAE-1 cable had been experiencing problems since February 2026 and was still awaiting repair six months later. The causes of the faults are usually mundane — fishing trawlers dragging anchors, undersea earthquakes, wear and tear — and ordinary repairs are slow.

The land routes and the cables coming next

Vietnam is not standing still. The long-term answer to cable fragility is redundancy: more routes, so that no single failure matters as much. Part of that answer is on land. Terrestrial fibre routes run through neighbouring countries to Hong Kong and Singapore, and in August 2025 VNPT added the Vietnam–Singapore Terrestrial Network, a land route through Laos connecting to data centers in Thailand, Malaysia and Singapore, with an initial capacity of 4 terabits per second and room to grow to 12. Land cables have their own risks, but they fail differently from sea cables, and that diversity is the point.

New submarine systems are also in the works, as regional demand for bandwidth keeps growing. Each new cable adds capacity and, just as importantly, a separate path, so a fault on one route no longer has to mean a national slowdown. The lesson of the last few years is that Vietnam’s internet is only as strong as its weakest cable, and the fix is not a single perfect cable but a web of alternatives, so that traffic always has somewhere else to go. The data centers being built across Vietnam depend on these links too.

What this means

Undersea cables are the part of the internet that Vietnamese users notice only when it stops working — which, lately, has been often. The country has six main systems, generous capacity on paper, and a history of simultaneous faults that expose how thin the margin really is. The short-term fix is what operators already do: reroute, balance loads and buy spare capacity when trouble hits. The long-term fix is redundancy on every level: more submarine cables, more land routes through neighbouring countries, and more content and services hosted inside Vietnam, so that a cable fault abroad does not slow down daily life at home. The goal is to make sure the next fault is an inconvenience, not a national event.