Almost All International Internet Traffic Travels on the Ocean Floor — Not Through Satellites

Cloud services and AI exist by exchanging data with the entire world. The physical layer carrying that traffic is the submarine cable network. This article explains what submarine cables are and why they shape data center siting. For the full picture of siting requirements, see Where can you actually build a data center? (in Japanese).

What is a submarine cable?

A submarine cable is a fiber-optic communications cable connecting continent to continent across the seabed. Surprisingly to many, the vast majority of international internet traffic runs through these cables, while satellite links carry only a tiny fraction.

When cloud services and AI move data across borders, most of it passes through these submarine cables. In other words, submarine cables are the backbone of the modern internet.

Why do submarine cables decide data center locations?

Response time (latency) is bound by physical distance. Light traveling through optical fiber has a hard speed limit, so the farther the distance, the longer the communication takes.

This is where the cable landing station — the facility where a submarine cable comes ashore — becomes critical. The closer a data center sits to a landing station, the faster and more stable its exchange with the rest of the world. As a result, the areas around landing stations and internet exchange points (IXs) become prime data center real estate. Alongside power and water, connectivity is a decisive siting factor (Data Center Dynamics).

What risks do submarine cables face?

Submarine cables are exposed to multiple risks: damage from fishing nets and ship anchors, undersea earthquakes, and — a growing concern in recent years — deliberate cutting (sabotage).

Where cables converge in a particular stretch of sea, or where a region depends heavily on a single cable, a cut causes major communications disruption. This has come into focus as a geopolitical risk in telecommunications, a security issue continuous with the broader geopolitics of semiconductors.

Where does Japan sit in the submarine cable network?

Japan is a critical junction for trans-Pacific cables and one of the hubs linking Asia and North America. NEC, meanwhile, is one of the world's leading players in submarine cable construction. For exactly where these cables come ashore in Japan, see Japan's Internet Comes Ashore at Just Two Places: Chiba and Mie.

This geographic advantage is one of the reasons Japan gets chosen as a data center location for Asia. Places that combine power, water, and connectivity are rare, and Japan satisfies several of those conditions at once. The power side of the equation is covered in AI data centers and the power crisis (in Japanese). This article is not investment advice.