The arrival of one of America's most advanced submarines in the Philippines' Subic Bay this month may not have generated many headlines across the region, but the significance was certainly not lost on Chinese officials and the mainland's burgeoning class of online military watchers.
As the Hawaii - a Virginia-class "hunter killer" attack submarine - moored alongside a specialist service ship, the Emory Land, it became the third US sub to visit Subic Bay since May - just one more sign of a growing US military presence across East Asia amid China's rise.
While the activities of surface warships - US aircraft carriers involved in exercises off Vietnam, or the dispatch of civilian Chinese patrol craft to the Diaoyu Islands, for example - routinely hog the headlines, it is the rarely-sighted submarines that are doing some of the most sensitive work as regional navies, some supported by the US, gear up to cope with China's military modernisation.
The nuclear-powered Hawaii is based in Pearl Harbour and called in at Subic for what US officials described as a routine rest and recreation stop. The precise nature of its long-term work beneath the waves of East Asia remains unclear, but it is perhaps significant that Subic is the closest traditionally friendly port for the US to the PLA navy's southernmost submarine base, built into the side of a cliff on Hainan Island at the top of the disputed South China Sea.
"When we see the US reasserting a presence at Subic with submarines, I fear we are seeing Washington's 'pivot' in action," said one Chinese envoy. "The pivot is not just words, it is already happening - and in the meantime we are left with the question of just how far they will go to contain China."
Outside of China, however, other questions are being raised as military strategists and analysts ponder how its navy will cope with the increasingly complicated undersea environment in its maritime backyard.
The Hawaii symbolises a shift that will see 60 per cent of America's submarines based in the Pacific by 2020 - a move that the commander of the US Pacific Fleet, Admiral Cecil Haney, said this week would also involve sending the most advanced US submarines in this direction.
The expanding US submarine presence in the region - centred on bases in Guam, Japan and Hawaii - is of course just part of the picture. Indonesia and Japan are expanding their fleets over the next decade while Vietnam - a nation with a strong military tradition - is due to take delivery of the first of six state-of-the-art Russian Kilo submarines by the end of the year.
The Russians, who also have ambitions to rebuild the strength of their Pacific submarine fleet, are helping Vietnam build submarine facilities at the highly-strategic Cam Ranh Bay on the South China Sea - a cold war-era Russian sub base.
US treaty allies Thailand and even the Philippines, a traditionally weak naval power, are considering their first submarine purchases.
However, one recent assessment of China's navy from US-based scholars Dr Andrew Erickson and Gabe Collins noted a lack of progress in China's anti-submarine warfare (ASW) capabilities, despite having a modern fleet of both nuclear-powered and diesel-electric submarines that US projections suggest will reach 46 vessels by the end of this year.
They outlined Chinese priorities that concentrated on China's near seas, with a more limited ability to project power beyond that - noting that "China's navy is not poised to speed across the Pacific to threaten America the way the Soviet Union once did, if not worse.
"To date, while it is conducting extensive research on acoustics and related areas, China has made little progress in ASW, and appears to avoid competing here for fear of wasting resources on immature and inadequate approaches," Erickson and Collins wrote in a survey published online.
"Its existing nuclear-powered submarines remain relatively noisy, though follow-on variants may be less so."
ASW covers a range of assets, from fixed sea-bed sensors to detect submarines, helicopters and planes equipped with advanced radars and sonar arrays to submarines themselves - as long as they are stealthy enough. ASW capabilities are particularly important to secure large, vulnerable targets such as aircraft carriers or convoys of transport ships in a time of conflict.
Gary Li, a PLA analyst with the London-based private-sector intelligence firm Exclusive Analysis, said that while the navy's development of its ASW capabilities had not seemed to keep pace with the expansion and modernisation of its other naval assets, China appeared to have considerable "breathing space" before regional navies would pose a significant threat.
"From the longer strategic point of view, regional submarine fleets are not going to be a threat within the next five to 10 years. So China's strategists probably think they have three or four years of breathing space before they have to seriously start expanding ASW capabilities."
"Right now, the focus seems to be elsewhere, judging by aggressive shipbuilding programmes that are under way," Li said.
"At this point, they seem content to let others expand their submarine programmes without being too worried."
While China would be able to produce Y-9 and Y-8 aircraft in large numbers for use on long-range anti-submarine patrols, a dedicated indigenous anti-submarine helicopter has not yet emerged - something that has surprised PLA analysts. For now, the navy still relied heavily on imported ASW helicopters such as the Ka-28, Li said.
Anti-submarine helicopters are a highly-specialised weapon. They need to be able to fly long distances and be able to carry and deploy sonar buoys to track enemy submarines, as well as torpedoes to sink them.
To understand the importance of anti-submarine warfare - and extreme sensitivities surrounding the region's expanding submarine programmes - it is useful to consider the nature of the work their crews perform.
In short, submarines routinely do the things that governments would rather not admit to.
The smaller hunter-killer attack subs can penetrate foreign countries' coastlines, going deep into territorial waters. They can perform a range of spying missions, from tapping undersea cables to eavesdropping electronically on targets on shore.
Exploiting stealth in times of conflict, a lone unseen hunter-killer, loaded with torpedoes and missiles, can routinely threaten much larger fleets of naval and merchant ships on the surface.
It is this deterrent factor that makes military analysts describe subs as "force multipliers" - an asymmetric weapon that allows smaller navies to make larger navies think twice.
Then there are the much larger ballistic and/or cruise-missile submarines, operated by just a few of the world's navies, that form the core of the strategy of nuclear deterrence.
The US still uses them, Russia is placing new ones in service, and China's Jin class vessels and missiles are expected to be fully operational within the next two years, according to the latest Pentagon estimates. China will have six Jin-class subs operational by 2016.
The vessels will be particularly vital to China's ongoing nuclear deterrent. As the only nuclear power to declare a "no first use" policy, the submarines are considered strategically vital in the worst case scenario - a future first strike destroying China's land-based missile silos.
The ballistic-missile submarines do not necessarily have to travel great distances, but they do have to be able to hide well to maintain their threat. That means avoiding being tracked by rival hunter-killers - a task the US can be expected to perform intensely in coming years, according to private military analysts. US undersea efforts to find, identify and track giant Soviet nuclear-armed submarines represented one of the key military rivalries of the cold war.
As well as working to raise the number and quality of submarines deployed in the Pacific, US military officials are attempting to work more closely with allied submarine powers, such as Japan, South Korea and Australia.
They are also attempting to discreetly support other submarine forces, such as that of Indonesia - home to some of the most strategic submarine chokepoints in the region linking the Indian and Pacific oceans.
Anti-submarine warfare is a key part of an exercise under way off Australia this month involving Australia, Brunei, Indonesia, New Zealand, Singapore and Thailand.
As one former US submariner explained, submarine activity across the region was reaching levels "not seen for decades.
"It's going to get quite crowded out there," he said.