[Japan’s Voices No.17] Protecting Japan’s International Submarine Cables

In July 2026, Japan’s Ministry of Internal Affairs and Communications issued a report on the protection of international submarine cables, concluding approximately six months of deliberations that began in November 2025. The nine-expert study group that prepared the report met seven times to assess emerging risks and propose countermeasures.

Prompting the establishment of this study group was a series of suspicious cable severings that occurred between late 2024 and spring 2025 in the Baltic Sea and around Taiwan. Submarine cables are occasionally severed by natural phenomena such as earthquakes or volcanic activity, or by accidental contact with bottom trawls or anchors. However, the incidents in question bore unmistakable signs of deliberate human interference rather than unavoidable accidents.

These suspicious severings were caused by large vessels—tankers and cargo ships—dragging their anchors across cable routes. The locations of submarine cables are typically indicated on nautical charts, and vessels are expected to avoid dropping anchors in such areas. Nevertheless, suspicious ships intentionally lowered their anchors in prohibited zones and continued dragging them along the seabed, sailing without weighing anchor. Although submarine cables are protected by polyethylene sheathing, the immense weight of an anchor—sometimes several tons—can strip away the outer layer, exposing internal metal components to seawater and rendering the cable inoperable.

Many of the suspicious vessels implicated in these severing incidents were registered in countries different from those where their owners reside. While numerous suspicious ships are operated by Chinese or Russian owners, their flags of registry are often those of third countries such as Cuba, Panama, the Cook Islands, or Belize. When a cable is severed within a coastal state’s territorial waters, that state may seize the vessel, detain the captain, and put him on trial. However, when the disruption occurs within an Exclusive Economic Zone (EEZ) or on the high seas, jurisdiction under the international law of the sea lies with the vessel’s flag state. If a cable is cut within Finland’s EEZ, for instance, the Finnish government cannot take any legal action if the offending vessel is registered in the Cook Islands.

Most vessels suspected of cutting cables claim the incidents were accidental, and in many cases no formal response is taken before they continue their voyage. 

Japan has not yet experienced such intentional severings in its surrounding waters. As an island country, though, Japan relies on submarine cables for 99 percent of its international communications. While low Earth orbit (LEO) satellite constellations such as Starlink may offer expanded capacity in the future, they cannot currently accommodate the massive data volumes carried by submarine cables.

Recognizing Japan’s heavy dependence on these cables, the Ministry of Internal Affairs and Communications organized the study group that produced a report whose central keywords are diversification and enhancement.

First, diversification is applicable to the routes of submarine cables. Submarine cable operators typically seek the shortest possible route to minimize latency. Nonetheless, practical considerations such as avoiding undersea volcanoes or fault lines and selecting suitable landing sites result in limited cable routes and concentrations of landing points in narrow areas. Cables connecting the US West Coast to Japan often land on the eastern side of the Bōsō Peninsula in Chiba Prefecture or in Ibaraki Prefecture. As concentration increases vulnerability, Japan must diversify both cable routes and landing points to reduce risks from terrorism, missile strikes, or other forms of attack.

Next, what is to be enhanced? In deep waters, submarine cables are directly placed on the seabed without any protection. Trans-Pacific submarine cables extend for about 9,000 kilometers, and it is impossible to continuously monitor such cable routes at the dark bottom of the sea. In shallower waters, the cables can be better protected by burying them or by armoring them with steel sheathing. Additional measures may include installing underwater sensors in vulnerable coastal areas and, in the future, deploying autonomous unmanned underwater vehicles (UUVs) for enhanced vigilance.

Maritime patrols must also be stepped up. When suspicious vessels linger near cable routes and their Automatic Identification System (AIS) signals raise concern, authorities must be able to swiftly proceed to the scene. NATO members have launched the Baltic Sentry initiative to jointly patrol and protect submarine cables in the Baltic Sea. Taiwan similarly maintains a blacklist of suspicious AIS signals and increases its vigilance when such vessels approach its waters.

Heightened protection must also be extended to terrestrial infrastructure. Cable landing stations require greater security and closer coordination with local police. Beach manholes, which provide cable access and are often located in publicly accessible areas, should be concealed or eliminated wherever possible.

The Ministry’s report presents measures that can be implemented immediately, while acknowledging that further work is needed. It proposes follow-up meetings by the end of the year to review progress and reinforce ongoing efforts.

For Japan, an island country, submarine cables are indispensable infrastructure. The security of submarine cables attracted limited attention during the post–Cold War era under the norms of an international community that embraced globalization. Today, however, geopolitical and geoeconomic realities demand more robust approaches. The protection of submarine cables can no longer be left solely to private operators. Japan must adopt security policies that address not only the “fog of war” but also the “murkiness of the seabed”.

Dr. Motohiro Tsuchiya is Professor at Keio University Graduate School of Media and Governance and Adjunct Fellow at the Japan Institute of International Affairs (JIIA). He is also one of the nine experts on the Japan’s Ministry of Internal Affairs and Communications’ Study Group on the Protection of International Submarine Cables.

The views expressed in this article are the author's and do not reflect those of JIIA CGO.