Executive Summary

Japan currently maintains localised subsea supremacy driven by electronic surveillance (ELINT) and surveillance capabilities, allied integration, and a high-quality submarine force. However, this is increasingly being challenged by China’s ambitious maritime modernisation and surveillance presence. At the same time, Japan is increasing its subsea cable dominance and strengthening its securitisation of relevant critical infrastructure. 

Japan’s subsea posture combines reliance on critical but vulnerable subsea cable infrastructure with a highly developed ISR architecture. This includes fixed listening arrays, airborne ELINT platforms, and close integration with U.S. systems. A small but technologically advanced submarine fleet completes this set. 

Consequently, these strengths enable effective monitoring of key chokepoints within the First Island Chain. Nevertheless, China is investing in oceanographic mapping, seabed sensors, autonomous underwater vehicles (AUV), and a growing, increasingly nuclear-powered submarine fleet. 

These developments are likely to erode Japan’s detection advantage over time. They also increase risks to both submarine operations and critical subsea infrastructure in the Indo-Pacific.

Key Judgements

KJ-1. Japan is moving toward a more securitised subsea cable architecture, integrating infrastructure protection with maritime domain awareness and ELINT-driven surveillance networks.

1. Approximately 99% of Japan’s international communications rely on subsea cables. The country operates more than 20 landing stations and approximately 30 cable systems, making it a critical Indo-Pacific connectivity hub. [source]
2. Japan subsidises domestic firms (e.g. NEC) to acquire dedicated undersea cable-laying and repair vessels to reduce reliance on foreign assets and improve crisis responsiveness. [source] 
3. According to information shared with Grey Dynamics by an insider source, the Japan Maritime Self-Defence Force (JMSDF) has at least 15 listening stations. These serve as shore terminals for the underwater arrays with ELINT capabilities, and sometimes with optical observation equipment. 

KJ-2. The JMSDF’s qualitative edge in anti-submarine warfare, largely due to advanced ELINT integration and surveillance network, enables effective tracking within the First Island Chain, despite China’s numerical advantage.

1. According to a US Navy source speaking to Grey Dynamics, the JMSDF benefits from deep interoperability with the US Navy. This includes shared undersea surveillance data and joint tracking operations. 
2. The JMSDF fields Kawasaki P-1 patrol aircraft, which integrate ELINT and networked data sharing. This provides high-end submarine detection and tracking capabilities. [source] 
3. Japan has 22-24 submarines, including Sōryū- and Taigei-class platforms, reflecting long-term qualitative investment in undersea warfare capability. [source]
4. A US Marine Corps officer who spoke to Grey Dynamics testifies to the qualitative superiority of Japanese submarine platforms in relation to Chinese counterparts. This is true particularly within the JMSDF’s current underwater surveillance architecture. 

KJ-3. China’s expanding seabed warfare ecosystem likely poses a growing threat to Japan’s submarine operations and subsea infrastructure security.

1. According to Reuters, China has deployed over 40 research vessels to map ocean floors and collect acoustic data. This directly supports submarine navigation, detection, and warfare effectiveness. [source]
2. RUSI research demonstrates that China is building an “Underwater Great Wall” of seabed sensors and unmanned systems to track submarines and enhance maritime domain awareness. These pose a real threat to allied systems in the region. [source] 
3. According to US Navy intelligence chief Rear Adm. Brookers, by 2035, China’s fleet may include more than 80 submarines, over half of which are expected to be nuclear-powered. This would substantially affect the power calculus in regional underwater security. [source]
4. At the September 2025 Beijing military parade, the PLA unveiled two new extra-large unmanned underwater vehicles (XLUUVs) the HSU100, and the AJX002. These feature ISR capabilities and also potential offensive payloads that can challenge coastal defenses, which make them a great risk for both underwater and China’s immediate waters in the Taiwan Strait. [source]

Statement on Analysis

This assessment is made with moderate-to-high confidence. Confidence is strongest in Judgements 1 and 2, supported by a mix of open-source reporting and defense-affiliated insights. However, some ELINT and seabed surveillance details rely on non-public sources and cannot be independently verified. Judgement 3 is held with moderate confidence due to its predictive nature and limited and often exaggerated reporting on Chinese capabilities.

Key assumptions include continued U.S.–Japan ISR integration, Japan’s focus on defensive operations within the First Island Chain, and sustained Chinese investment in seabed warfare and submarine modernization. Simultaneously, gaps remain on the coverage and resilience of JMSDF underwater sensor networks and the true acoustic performance of newer Chinese submarines.

This assessment could change with evidence of Chinese sabotage of subsea infrastructure or deployment of operational seabed sensor networks. Further, shifts in regional maritime operations initiated by China against Japan after PM Takaichi’s more diplomatically assertive stance against the former may also impact our analysis.

Intelligence Cut-off Date: 17 April 2026