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Silent dispute at the bottom of the Pacific shows how submarine sensors, hydrophones, and autonomous drones have begun to influence the military balance between China, Japan, and the United States, in a region marked by strategic routes, naval bases, and high-range technological surveillance.
In the western Pacific, the military dispute has taken on a decisive dimension below the surface, where sensors, hydrophones, and coastal stations help Japan and the United States monitor the movement of Chinese submarines in strategic areas.
This pressure became more evident on September 3, 2025, when the People’s Liberation Army showcased new unmanned underwater vehicles during a military parade held in Beijing.
Described in 2015 by Desmond Ball and Richard Tanter, researchers affiliated with the Australian National University, the logic of this surveillance appeared in the book The Tools of Owatatsumi, published by ANU Press.
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The work analyzed Japan’s maritime monitoring capability, with a special focus on the submarine segment, precisely an area on which there was little strategic analysis available to the public until that moment.
Submarine Surveillance in the Western Pacific
According to the study, Japan maintains an extraordinary network of underwater hydrophones connected to land stations, capable of enhancing listening capabilities in internal straits and nearby seas.
These bases can also operate electronic intelligence systems used to monitor, identify, and track submarine and surface traffic, creating an additional layer of surveillance in sensitive routes of the western Pacific.
In part, this structure is jointly operated with the United States Navy and holds strategic importance for Washington, according to ANU Press.
In practice, the network functions as a surveillance barrier between the East China Sea, the South China Sea, and the paths leading submarines to the open Pacific.
Hook Submarine Defense Line
Known as the Hook Submarine Defense Line, the architecture received this name due to the geographical shape attributed to the sensors distributed across essential maritime passages.
The network would start in the Tsushima Strait region, between Japan and Korea, advance through areas near Taiwan and the Philippines, and connect to sections further south in the Indo-Pacific.
In its operational logic, the system resembles SOSUS, the underwater listening network used by the United States during the Cold War to track Soviet submarines over long distances.
In the current scenario, however, the strategic target has become the Chinese naval expansion and Beijing’s ability to deploy submarines to the open ocean without prior detection.
Ball and Tanter assessed that the Japanese advantage in submarine detection could be decisive in a confrontation of this nature, especially in regions that are mandatory passages for Chinese submarines.
At the same time, the researchers warned that fixed and isolated installations also become vulnerable points, as they concentrate critical surveillance and command functions in high military value locations.
This delicate balance helps explain why Beijing has started investing in means capable of operating beneath the surface, without relying solely on conventional submarines or patrol ships.
For China, underwater drones and autonomous vehicles represent not only offensive power but also an attempt to reduce dependence on routes monitored by United States allies.
Underwater Drones Displayed by China
During the parade on September 3, 2025, China presented two classes of unmanned underwater vehicles, according to the Asian Military Review.
Among the equipment displayed, the highlight was the HSU100, described as an extra-large autonomous underwater vehicle, accompanied by the AJX002, another large system shown in the same parade.
The publication reported that the HSU100 appeared with retractable sensors, likely aimed at intelligence, surveillance, and reconnaissance operations in underwater environments.
Based on the size observed in the parade, the vehicle occupied a large part of the trailer on which it was transported and was estimated to be about 15 meters long.
The AJX002 was described as a vehicle with a smaller diameter but with a length similar to the HSU100, indicating a different configuration for undersea missions.
Without visible retractable sensors, its cleaner design reinforced the perception that Beijing is developing distinct solutions for underwater tasks, without publicly revealing all technical details.
These systems do not, by themselves, prove the existence of a Chinese “wall” on the seabed, nor do they allow for the assertion that Beijing has already breached adversary surveillance.
Even so, the public display of underwater drones indicates that China is attempting to expand its ability to operate in an environment where Japan and the United States already had a technological advantage described a decade earlier.
Military Balance in the Indo-Pacific
With the advancement of submarine surveillance, the pressure on Beijing has shifted from being merely technical to directly influencing the regional military balance.
When a power can track rival submarines near bases and exit routes, it reduces the adversary’s operational freedom and increases its own response capability.
ANU Press summarizes this point by stating that Japanese submarine detection capabilities offer a significant advantage but also create escalation dynamics.
In a crisis, both sides might feel the need to preserve or neutralize systems considered essential before the situation spirals out of control.
In this scenario, Taiwan, the Philippines, the Tsushima Strait, and the South China Sea emerge as areas of passage, dispute, and constant monitoring.
The importance of these points lies not only on the map but in the ability to control what crosses maritime corridors used by ships, submarines, and patrol forces.
More than a response to an isolated incident, the Chinese reaction reflects an accumulated shift in the Indo-Pacific over the last decade.
The advancement of allied sensors on the seabed and the display of Chinese underwater drones show that naval competition has shifted to a less visible dimension, yet central to the military balance of the region.
In this environment, control of the seabed has become as significant as aircraft carriers, missiles, and coastal bases in the projection of military force.
Instead of relying solely on surface ships, the involved powers invest in listening, tracking, electronic intelligence, and autonomous vehicles to anticipate movements before they appear on the horizon.
