It's been a gruesome mystery for years: the wrecks of wooden boats crewed only by skeletons found adrift in the Sea of Japan, which is also known as the East Sea.
But these 'ghost ships' have become a macabre spectre: More than 150 washed ashore last year alone. Some are split in half. Others are empty, but eerily intact. Some carry dead crews. A few hold steadfastly silent survivors. All were clearly North Korean.
Japanese authorities assumed the poverty-stricken fishers had sailed too far for too long in a desperate hunt for increasingly scarce fish. Or that they were defectors from North Korean leader Kim Jong Un's authoritarian regime.
Now NBC News and a report by the University of Wollongong has exposed the presence of a vast, anonymous "Dark Fleet" operating in North Korean waters. And it's been directly linked to Beijing.
UNDER THE RADAR
A study published in the journal Science Advances this week exposes a 'Dark Fleet' – fishing trawlers with their identity and location transponders turned off – operating in the Sea of Japan (East Sea).
Like the South China Sea, it is a contested waterway. North Korea, South Korea, Russia and Japan disagree over who owns which patch of water. Which makes policing the area fraught with diplomatic risk.
But now the movements and identity of the 'Dark Fleet' is being illuminated by modern technology.
"By synthesising data from multiple satellite sensors, we created an unprecedented, robust picture of fishing activity in a notoriously opaque region," says study co-author Jaeyoon Park.
"The scale of the fleet involved in this illegal fishing is about one-third the size of China's entire distant water fishing fleet. It is the largest known case of illegal fishing perpetrated by vessels originating from one country operating in another nation's waters."
The trawlers were detected leaving Chinese harbours. They were tracked passing through the Korea Strait. They were identified operating in North Korean waters.
"The massive scale of this illegal operation poses substantial implications for fisheries governance and regional geopolitics. If the vessels are not approved by their flag State (China) and the coastal State (North Korea), then they are fishing illegally," says University of Wollongong associate professor Quentin Hanich.
"This analysis represents the beginning of a new era in ocean management and transparency. Global fisheries have long been dominated by a culture of unnecessary confidentiality and concealment."
But the NBC News team found aggression is also now part of that culture.
"Reporters for this article filmed 10 of these illegal Chinese fishing ships crossing into North Korean waters. However, the reporting team was forced to divert its course to avoid a collision after one of the Chinese fishing captains suddenly swerved toward the team's boat," the NBC report reads.
"Spotted at night and roughly 100 miles (160km) from shore, the Chinese squid ships would not respond to radio calls and were travelling with their transponders off."
FIGHTING FOR FISH
It could be a sign of things to come.
In 2009, a swarm of trawlers encircled and harassed the USS Impeccable, a US survey ship.
In 2016, a South Korean Coast Guard cutter was rammed and sunk while attempting to expel a group of Chinese trawlers from its territorial waters.
In June last year, the crew of a Philippine fishing boat was abandoned to the sea after it was rammed and sunk by a Chinese trawler at night.
In June this year, a Chinese Coast Guard vessel rammed a Vietnamese fishing boat in the South China Sea's Paracel Islands. A similar incident in April saw a Vietnamese ship sunk.
Beijing is increasingly asserting its unilateral territorial claim to almost the entirety of the East and South China seas. Earlier this year, that claim reached as far south as Indonesia – with a Chinese fishing fleet supported by navy-controlled coast guard vessels intruding on Natuna island.
And it's ignoring international complaints.
In 2016, an arbitration tribunal for the law of the sea found Chinese vessels had been unlawfully blocking Filipino fishers from their traditional grounds at Scarborough Shoal.
The same tribunal accused the Chinese of destructive fishing methods, destroying the habitat of giant clams and vital coral reef spawning grounds. Many of the region's most significant ecological habitats have been dredged up and buried under concrete to form Beijing's illegal island fortresses.
Now, NBC News network says "famously aggressive, often armed" Chinese trawlers are muscling in on North Korean waters.
"Competition from the industrial Chinese trawlers is likely displacing the North Korean fishers, pushing them into neighbouring Russian waters," researcher Jung-Sam Lee told NBC. They're also trying to reach Japanese waters, with 2000 intercepted in 2017 alone.
And that means North Koreans are being forced further out to sea than ever before, often beyond the capabilities of their basic timber craft.
Once in North Korean waters, the Illuminating Dark Fishing Fleets report says the Chinese trawlers show no regard for sustainable fishing practices.
Not only are they scaring North Koreans out of the fishing grounds, they're stripping them bare.
And this has a direct link to the discovery of 'ghost boats' washing up on Japan's beaches.
"Many fishing villages on North Korea's eastern coast have now been coined 'widows' villages'," the researchers write.
The 'Dark Fleet' is believed to have pulled 160,000 tonnes of squid from the region in just one 2018 fishing season.
And that has serious implications.
Pacific flying squid catches have collapsed by more than 80 per cent in South Korean and Japanese waters since 2003. This squid is one of the top five kinds of seafood eaten in Japan. It's also South Korea's most crucial ocean harvest. And it's a vital staple for the diet of impoverished North Korean coastal towns.
Beijing denies it has any knowledge of the fleet's origins or activities. But this strikes a discordant note among international analysts. China's fishing fleets are tightly controlled. They are formed into 'militias' and co-ordinated by Communist Party political officers. They are required to work in close concert with the nation's navy.
The Sea of Japan (East Sea) isn't the only location 'dark' Chinese fishing fleets have been observed. A Centre for Strategic and International Studies (CSIS) report from 2019 similarly tracked trawlers' going dark' in the South China Sea.
"As they race to pull the last fish from the South China Sea, fishers stand at least as much chance of triggering a violent clash as do the region's armed forces," the report reads.
But it also identified a more ominous behaviour.
Many of the fishing boats are not fishing. They're asserting control.
"A different kind of fishing fleet, one engaged in paramilitary work on behalf of the state rather than the commercial enterprise of fishing, has emerged as the largest force in the Spratlys," the CSIS study finds.
"The activities of the militia are well-documented — they engage in patrol, surveillance, resupply, and other missions to bolster China's presence in contested waters in the South and East China Seas. Beijing makes no secret of their existence, and some of the best-trained and best-equipped members engage in overt paramilitary activities such as the harassment of foreign vessels."
These non-fishing trawlers usually congregate around contested reefs and over disputed fishing grounds. Their presence alone forces non-Chinese fishers to move on.
And the spate of recent collisions and 'incidents' in the South China Sea indicates they are getting more and more assertive.
Last year, the United States declared it was "aware" China's fishing fleet was being used to assert territorial control. Then Chief of Naval Operations Admiral John Richards warned these trawlers could be considered naval auxiliaries because of the military nature of their training and operations.
"In the run-up to a conflict, the maritime militia may employ coercive tactics, such as ramming vessels to goad an adversary into striking back, while CCG and even PLAN forces wait over the horizon to rush to the scene and 'teach a lesson'," warns Professor James Kraska of the US Naval War College.
The alternative is a clear victory for Beijing.
"Failing to confront the (fishing militia) normalises Beijing's presence and reach in other nations' territorial seas and EEZs."