Mysterious, crew-less "ghost ships" have been washing up on the western shores of Japan.
Over the past two months at least 13 wooden boats have turned up, with more than 20 decaying bodies on board. Very little is known about them but investigators have found some evidence that hint at their origins.
What are these 'ghost ships'?
They are so called because they have been found empty or with only corpses on board off Japan's western coast, stretching from Fukui prefecture to the southern tip of Hokkaido . All the bodies were either decomposing or partially skeletonised by the time they arrived, a clear indication they had been dead for a long time.
But this is not the first time boats have come ashore in Japan or on the coast of Russia's far east. Japan's coast guard told the BBC that 65 such boats washed up last year, but the latest influx appears to be coming at a slightly higher rate than usual.
Where are they from?
They are believed to be North Korean fishing boats, many of which will have been out searching for king crab, squid and sandfish at this time of year. Markings on at least one of the boats, in Korean, indicated that it belonged to the North's military.
In North Korea the military is heavily involved in the fishing industry, as it is in many others.
A scrap of what is thought to be part of the North Korean flag flying from one of the boats is also a clue. Unsurprisingly, there has been no mention of the missing vessels from North Korea.
How did those on board die?
Not every boat has been found with corpses.
Japanese officials are investigating the causes of death but say some of the bodies are in such a bad state of decomposition that it may be impossible to identify cause of death.
It is now winter in the region and with little food on board, exposure and starvation are possible explanations too.
Japan normally bans North Korean ships from landing in the country, although it makes exceptions on humanitarian grounds, such as for ships sheltering from storms.
Are they defectors?
Some commentators have suggested that purges could be behind this, speculating that sailors could be trying to flee the regime. There have also been reports of tighter control of the North Korea-China border, the most common route for defectors.
But many remain unconvinced.
Dr John Nilsson-Wright, head of the Asia Programme at think-tank Chatham House told the BBC that aside from the cultural and linguistic ties, "it wouldn't make sense if you were a defector to go to Japan. South Korea is much closer by boat."
So what happened to these boats on the open sea?
The wooden boats arriving are old and heavy and have neither powerful modern engines nor GPS navigation systems. If they ventured too far out or were blown off course, they could lose their bearings or find it hard to beat the currents even if they knew which way to go, analysts say.
The fact that it is relatively common for these boats to appear also suggests that this, not purges in Pyongyang, might be a more likely explanation.
Weather is unlikely to be a factor. While the Sea of Japan had rougher seas and stronger winds in November, the Japan Coast Guard told the BBC these were normal conditions.
Why would fishermen take this risk?
ne hypothesis is that the leadership is demanding bigger catches, and they have been forced to take chances to meet their targets. State TV has shown Kim Jong-Un at fishing facilities, exhorting the country to boost production.
But the leader's media appearances have not convinced everyone.
"Agricultural yields seem to be up," Dr Nilsson-Wright says, suggesting that profit-seeking would be a more plausible incentive to take such risks.
It is common in North Korea for workers to keep some of the surplus they generate past the targets set by the state. This quasi-capitalist system has been credited with improving production, analysts say.
But, if you are especially poor, as many North Koreans are, "you will do anything you can to improve your own existence", says Dr Nilsson-Wright.
This could include taking desperate chances at sea: "It could simply be that they were just unlucky."