NTT Group's submarine cable-laying vessel Kizuna, Nagasaki.
Undersea communication cables form the backbone of global telecommunications, serving as an indispensable part of modern infrastructure. In recent years, a series of suspected sabotage incidents around the world has underscored the growing importance of protecting these critical networks.
Japan, an island nation connected by hundreds of remote outposts, relies on roughly 500 domestic submarine cables spanning about 18,000 kilometers. The task of maintaining and repairing these vital lifelines falls to NTT Group's cable-laying vessel, Kizuna. In mid-October 2025, this reporter boarded the ship under the guidance of Captain Atsushi Sakurai, while it was moored at its home port in Nagasaki.
A Ship the Height of an Eight-Story Building
The Kizuna is an 8,598-ton vessel measuring 109 meters in length and 20 meters across. It operates throughout the Pacific Ocean, Sea of Japan, and Philippine Sea. With its bridge and crew quarters positioned toward the bow to make room for equipment loading at the stern, the ship has a distinctive, almost truck-like profile. From keel to mast, it stands roughly as tall as an eight-story building.
On deck, a several-meter-wide pit allows cables to be hauled up from the storage hold below. Looking down, the floor appears a dizzying ten meters beneath one's feet. The deck can also be sealed or opened as needed, enabling the vessel to transport relief supplies such as mobile base stations in times of disaster.
Cable Handling: A Tough, Manual Task
Below deck, Kizuna houses two massive reels capable of storing a combined 4,000 kilometers of cable. Loading them is a delicate process that requires heavy cranes and careful coordination. To prevent twisting or damage to the repeaters, more than a dozen crew members work by hand, guiding each coil into place.
Life aboard the vessel can be demanding.
"Once, during winter in the North Pacific, we ran into a storm and couldn't work for months," recalled Captain Sakurai. "When we operate far from shore, a single voyage can last more than a month. Hauling a cable from several thousand meters below takes over a day."
Each cable is only a few centimeters thick, its optical fibers protected by a polyethylene sheath. In shallow waters, layers of steel wire are added for reinforcement, but in the deep sea, where immense pressure poses its own hazards, international cables remain largely unarmored and exposed.
Lives at Risk
Most undersea cable failures are the result of human activity — primarily trawling nets and ship anchors. According to a study by the Mitsubishi Research Institute, 40.8% of damages stem from fishing operations, 15.8% from anchors, and 9.5% from deliberate acts such as piracy.
Captain Sakurai said he has not personally observed any rise in military threats within Japanese waters. Coastal areas, he noted, are under close watch by fishers and maritime authorities. Locating specific cables on the high seas remains extremely difficult without detailed data.
Even so, recent cases in the Baltic Sea and around Taiwan, believed to involve sabotage, have underscored the growing risks. "The importance of maintenance and protection can't be overstated," Sakurai said.
At a recent gathering, a friend from Japan's Maritime Self-Defense Force offered a half-joking warning:
"If anyone wanted to disrupt communications, they'd target the cable-laying ships first. Your life could be at risk."
Sakurai laughed it off as a lighthearted comment over drinks, but admitted that he has since told his family, "If anything ever happens to me, you'll know why."
Nagasaki: Japan's Link to the World
In Nagasaki, where the Kizuna is based, a modest stone monument stands quietly along the slope leading to Oura Church. It marks the birthplace of Japan's international telegraph. The country's first undersea cable, laid in 1871 by a Danish telegraph company, connected Nagasaki with Shanghai. It was later used by the British military.
From the Meiji era (1868-1912) to today, Japan's history has shown that communication and national security are inseparable.
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(Read the article in Japanese.)
Author: Katsutoshi Takagi, The Sankei Shimbun
