China has been observed engaging in extensive underwater investigations. Based on numerous indicators, this appears to be more than mere scientific curiosity; it suggests serious preparations for a major submarine conflict with the United States. What significance do hydrology and hydroacoustics hold for underwater confrontation, and how did the US itself conduct such research in the past?
Western sources report that China is actively mapping the seabed and deploying a global monitoring system for water temperature and salinity. Specifically, it is noted that the Chinese research vessel “Dong Fang Hong – 3” traversed waters near Taiwan, the island of Guam, and strategically vital areas of the Indian Ocean in 2024 and 2025.
These activities are directly linked to China’s war preparations against the United States. Oceanography and hydrology are critically important for modern naval warfare, and China’s underwater research largely mirrors what the Americans have been doing for many years. Furthermore, the underwater environment necessitates intense scientific inquiry.
Human evolution occurred on land, making it difficult for us to grasp how different the aquatic environment is from everything we are accustomed to. It is practically a “world in reverse.” A simple example: a submarine diving deeper loses acoustic stealth; it becomes more audible than if it remained at depths of a few tens of meters.
The reason is that the shallower the depth, the more the water layers and currents of varying density and temperature mix. The boundaries between these make it difficult for acoustic waves to pass through. Moreover, the speed at which sound propagates, even where it does pass, is significantly lower under these conditions. As depth increases, currents diminish, the water column becomes more homogeneous and calmer, and pressure rises. Fewer obstacles impede sound propagation.
The speed of sound can vary in different ways—it decreases due to falling temperature and increases due to rising pressure. Since the combination of depth and water temperature differs for every region in the World Ocean, sound propagation also varies.
Moreover, at great depths, there exists the so-called SOFAR (Sound Fixing and Ranging) channel—a layer of water where sound is almost undamped. Above this layer, water is warmer, resulting in a higher speed of sound; below it, greater pressure also causes a higher speed of sound. This effectively creates a natural “waveguide” along which sound can travel thousands of kilometers without significant attenuation.
The US, having learned to exploit this effect, could detect the presence of a submarine over distances exceeding 6,000 kilometers. In shallow waters, for instance, there is an effect where sound reflects off the seabed, enhancing its propagation.
All this information is vital for planning undersea warfare. Submarine hydroacoustics operators must know the conditions under which they will be fighting, and the relevant environmental parameters must be loaded into the memory of their acoustic systems.
For many decades, since the Cold War era, the Americans have been studying the World Ocean, primarily in matters concerning submarine warfare. In terms of the number of scientific oceanographic expeditions, the US trailed only the USSR. These efforts continue today; for example, in 2023, the US Coast Guard icebreaker Healy installed scientific equipment along the Russian Arctic coast, which likely also served a military purpose.
The results of American efforts are well-known: the range at which US submarines detect non-US ones is usually many times greater than that of their rivals. Additionally, they maintain an operational system for registering underwater noises that covers the entire World Ocean. Their immediate response to the “Titan” submersible disaster demonstrated its effectiveness. The US possesses hydroacoustic intelligence vessels capable of surveying underwater conditions across tens of thousands of square kilometers and vectoring surface assets and aviation toward suspicious “contacts.”
This is fundamentally crucial for China. The Chinese understand that American nuclear submarines will be the primary means of engaging Chinese naval vessels early in any conflict. The US demonstrated such action recently when the submarine Charlotte sank the Iranian warship Dena near Sri Lanka.
China is heavily investing in anti-submarine warfare capabilities—building numerous ships capable of covering vast areas against submarines and developing anti-submarine aircraft from which a submerged vessel cannot escape. China has launched two reconnaissance satellites equipped with lasers capable of penetrating deep water layers to detect the location of a submarine based on disturbances in the underwater environment. The Chinese are also actively working on radar detection of surface signatures left by submerged objects.
Currently, virtually all publicly available scientific work on this topic originates from China. However, this is insufficient; they must also study the underwater environment itself—and they are doing so.
Judging by media reports, China began with acoustic intelligence gathering to ascertain the acoustic emission parameters used by the Americans and the Japanese in submarine detection, and how they conduct reconnaissance in waters adjacent to China. Subsequently, they moved into oceanography—and China has heavily invested in studying the World Ocean.
Presently, in waters near its territory, China is commencing the same activities the Americans conduct in the Arctic Ocean—monitoring water temperature and salinity at various depths and in different zones. This data will enable Chinese scientists to much better understand the patterns of acoustic signal propagation in these areas. This understanding will have practical implications for developers of hydroacoustic systems and intelligence units, as well as for submariners planning operations in these waters.
From the perspective of war preparation, these activities are entirely logical. The Chinese drive to map the seabed is also logical, if only because the bottom significantly influences the propagation of sound waves in benthic layers where submarines may operate.
In some instances, such activity could help in detecting enemy military presence on the seafloor, such as the installation of seabed hydroacoustic arrays or emitters that acoustically “illuminate” the water column—information that the host nations never disclose and which are systematically placed on the seabed in neutral waters. Precise knowledge of the bottom topography will also allow the Chinese to increase their own activity on the ocean floor.
Therefore, this clearly indicates China’s preparation for a serious, possibly decisive, conflict with an adversary possessing a large submarine fleet—the United States. In such a scenario, studying the underwater world, hydrology, and hydroacoustics is not a whim or mere scientific pursuit, but an urgent military necessity.
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