How Submarine Cables Work: Your Videos Cross the Ocean Floor, Not Space

It is easy to assume the internet is connected through the sky, via satellites. In reality, the ocean floor does almost all the work. This article walks through what submarine cables are, who builds and owns them, and where they come ashore in Japan.

What Is a Submarine Cable?

A submarine cable is a fiber-optic communications cable that connects continents across the ocean floor. Submarine cables carry almost all intercontinental internet traffic; the share carried by satellites is tiny. TeleGeography notes that while a precise figure is hard to calculate, satellite traffic accounts for only a small fraction of international capacity.

There are roughly 600 submarine cables in service or under construction worldwide, with a combined length that would circle the Earth many times over. When cloud services and AI move data across borders, the vast majority of it travels through these cables. Submarine cables are, quite literally, the backbone of the modern internet (TeleGeography).

Why Do Cloud and AI Need Submarine Cables?

Cloud services and AI both depend on exchanging data with data centers around the world, and submarine cables are the physical infrastructure carrying that traffic. Because response time (latency) is bound by distance, the areas near landing stations — where cables come ashore — are prime locations with fast, stable connectivity to the rest of the world.

Alongside power and water, connectivity is one of the conditions that determines where data centers get built. The full siting picture is covered in Where can data centers be built? (in Japanese), and the relationship between cables and siting in Submarine cables and data center siting.

How Do Netflix and Streaming Video Actually Travel?

Surprisingly, most popular videos never cross a submarine cable. Netflix and YouTube (Google) use a mechanism called a CDN, pre-positioning frequently watched videos on servers inside each country. Because playback does not need to fetch content from the US every time, streams are fast and the submarine cables stay uncongested.

How this in-country caching works, and how OTT providers (companies that deliver video and other content over the internet) economize on submarine cable capacity, is covered in detail in How Netflix and YouTube deliver video (in Japanese).

Who Builds Submarine Cables, and Who Owns Them?

Effectively only four major manufacturers can build submarine cables: ASN (France), SubCom (US), NEC (Japan), and HMN (China). And in recent years, ownership has shifted from consortia of telecom carriers to hyperscalers like Google and Meta. The share of international capacity used by the four biggest players jumped from about 10% in the early 2010s to over 71% in 2024.

Who builds them, who owns them, and how fast the market is growing are all covered in Only Four Companies in the World Can Build Submarine Cables (Verdict / TeleGeography).

Where Do Cables Come Ashore in Japan?

Most trans-Pacific cables reaching Japan land in just two areas: Chiba Prefecture (Chikura and Maruyama) and Mie Prefecture (Shima). The reasons are geography — these points offer the shortest path to the US — and redundancy: two separate east-west corridors so a single disaster cannot take everything down at once.

The deep-sea section of a cable is only 17-21mm in diameter — about as thick as a thumb. Packed into that slender core, in concentric layers, are the optical fibers that carry the traffic plus the power-feeding and protective layers around them. A map of Japan's landing stations and a look inside the cable are covered in Where do submarine cables land in Japan?, with maps and 3D models.

Where Should You Start Reading?

To go deeper on submarine cables, we recommend this order:

1. How Netflix and YouTube deliver video (in Japanese): how CDNs shoulder the submarine cables' load.
2. Only Four Companies in the World Can Build Submarine Cables: the four-company manufacturing oligopoly, the hyperscaler ownership shift, and market size.
3. Where do submarine cables land in Japan?: a map of landing stations, plus cable thickness and structure.

Key Takeaways

• Almost all intercontinental traffic travels over submarine cables, not satellites.
• Netflix and YouTube cache popular videos on in-country servers (CDNs), so most streams never cross a submarine cable.
• Only four companies can effectively build the cables: ASN, SubCom, NEC, and HMN. Ownership is shifting to Google and Meta.
• In Japan, trans-Pacific cables land overwhelmingly in two areas: Chiba (Chikura and Maruyama) and Mie (Shima).
• The deep-sea section is just 17-21mm in diameter — thumb-thin — yet packs optical fibers plus power and protective layers.

This article is not investment advice.