Japan's Internet Comes Ashore at Just Two Places: Chiba and Mie

Every international connection must come out of the sea and onto land somewhere. That doorway is the cable landing station. Once you know where Japan's landing stations are, you can also see why data centers cluster in particular places. For the broader picture, see the complete guide to submarine cables.

What is a cable landing station?

A cable landing station is the facility located where a submarine cable comes out of the sea onto land. The cable that has run across the seabed connects to terrestrial networks here. Landing stations are the gateways of international connectivity — the closer you are to one, the faster and more stable your exchange with the world.

A landing station houses power-feeding equipment that sends electricity into the undersea cable, along with equipment that conditions the signals. These are unassuming seaside buildings, but they are keystone infrastructure for international communications.

Where do cables come ashore in Japan?

Plot Japan's main landing stations on a map and they cluster at a handful of points along the Pacific coast.

The most important are Chikura and Maruyama in Minamiboso City, Chiba Prefecture, and Shima in Mie Prefecture. These form Japan's two great gateways: Chikura hosts FASTER and SJC2, Maruyama hosts JUPITER (Meta, Amazon, NTT and others) and ADC, and Shima hosts Topaz (Google's cable to Canada) and FASTER, among other major systems. Kitaibaraki in Ibaraki Prefecture was once a main hub for Japan–US cables, but the Japan-US Cable Network was retired in 2023 and the site is now mostly legacy systems (the Ibaraki end of Topaz lands in neighboring Takahagi City). Okinawa serves as a branching point toward destinations across Asia. Ninomiya in Kanagawa Prefecture was a historic landing station but has since been demolished (Submarine Networks).

Why do cables concentrate at Chikura and Shima?

There are two reasons. The first is geography. The Boso and Shima peninsulas offer the shortest way out into the Pacific and a favorable route toward North America. A peninsula jutting into the sea lets a cable reach deep water by the shortest path.

The second is redundancy. Concentrate everything in one place, and an earthquake, tsunami, or cable cut could take the whole system down at once. So the network is split into an eastern leg (Chiba) and a western leg (Mie): if one side is hit, the other keeps traffic flowing. Why proximity to landing stations shapes siting is covered in submarine cables and data center siting, and the full set of siting requirements in Where can you actually build a data center? (in Japanese) (Submarine Networks).

How much capacity reaches each station?

An important caveat first: the actual traffic flowing through each station is not disclosed. The useful proxy is the design capacity of the cables landing at each station — the theoretical maximum throughput, effectively the "thickness" of the pipe. Adding these up gives a sense of each station's scale.

Tbps (terabits per second) measures data volume per second; 1Tbps is enough to stream tens of thousands of high-definition videos simultaneously. As the table shows, capacity is dominated by three stations — Shima, Maruyama, and Chikura — each in the hundreds of Tbps. Shima (about 440Tbps class) is the largest single station, but at the prefecture level, Chiba (Maruyama plus Chikura, about 665Tbps class) is Japan's largest. The recent pattern is that the newest high-capacity cables — Topaz in 2023, ADC in 2024, SJC2 in 2025 — keep converging on these three stations, while the former mainstays of Kitaibaraki and Okinawa now handle mostly legacy or regional traffic (Submarine Networks / TeleGeography).

How thick is a submarine cable?

Surprisingly thin. The deep-sea sections measure just 17–21mm in diameter — about a garden hose, or a human thumb. It is striking that the great arteries of global communications are lines this slender.

Near shore, in shallow water, the situation changes. Damage from anchors and fishing nets is far more likely, so the cable is wrapped in steel-wire armoring, thickening it to a maximum of about 50mm. In other words, the cable changes its thickness to match its environment.

What is inside the cable?

The cross-section of this slim cable is a set of concentric layers, from the inside out:

• Optical fiber bundle: the innermost core and the actual communications medium. Each fiber is about as thin as a human hair, bundled in several to a dozen-plus pairs.
• Steel strength wires: the core that withstands laying tension and deep-sea pressure.
• Copper conductor: the path carrying electricity from shore to the repeaters — a high-voltage DC feed of 3–15kV.
• Polyethylene insulation: a thick resin layer that insulates the high voltage and seals out seawater.
• Armoring (steel wires): the outer wrap used only in shallow water; omitted in the deep sea, where the cable slims down considerably.

Why send electricity at all? Because optical signals fade over long distances, repeaters must be placed roughly every 50–100km to amplify the signal — and the power to run those repeaters travels from shore through the copper conductor. Per-cable capacity is also enormous: recent trunk systems reach levels like 16 fiber pairs carrying 240Tbps (the Topaz example). A line the width of a thumb performs all of these roles at once (Hackaday).

Key takeaways

• Trans-Pacific cables reaching Japan land overwhelmingly in two areas: Chiba (Chikura and Maruyama) and Mie (Shima).
• The reasons are geography — the shortest route to the US — and redundancy: splitting into eastern and western legs so a single disaster cannot take both down.
• Design capacity is dominated by Shima (about 440Tbps class), Maruyama (about 400Tbps class), and Chikura (about 265Tbps class); new cables (Topaz, ADC, SJC2) concentrate there, while actual traffic is not disclosed.
• Deep-sea sections are 17–21mm in diameter (about a thumb's width); armored shallow-water sections reach about 50mm.
• From the inside out: optical fiber, steel wires, copper conductor, insulation, armoring. Shore stations feed 3–15kV to repeaters that amplify the signal every 50–100km.