Nearly 100 dilapidated wooden boats have washed up in northern Japan this year. Sometimes they’re empty, other times they carry corpses, or skeletons, earning them the eerie title of “ghost ships”. And it’s thought they’re coming from North Korea.

The influx of these vessels into Japan’s waters has become something of a seasonal occurrence, as autumn and winter winds push dozens of the boats toward the archipelago from the west. This time last year, VICE News reported that 44 ghost ships had washed up around Japan over the course of 2017. In 2016, that number was 66.

This year, Yomiuri Shimbun is reporting that a record number of 89 vessels, either wrecked or deserted, have been identified near Japanese shores, according to the Tokyo coast guard. Many have been in the vicinity of the Hokkaido region to the north, and almost all of them on the country’s western flank facing the Korean Peninsula. Of those 89 vessels, five were found to contain a total of 12 dead bodies.

Analysts have suggested that a number of the boats were probably carrying refugees who were driven out to sea by deteriorating social and political conditions in North Korea, VICE News previously reported. Another popular theory is that the ghost ships were crewed by fishermen struggling to fulfil fishing quotas. North Korean fishermen are under intense state pressure from the Kim Jong Un regime to bring in more fish and boost protein sources in the country, according to United Press International, and are likely being forced to take more risks and venture further out to secure a catch.

In any case, the annual inrush of vessels from North Korea has raised security concerns in Japan, prompting authorities to step up their patrols for wayward boats and their crews—dead or alive. While the majority of the vessels are either unmanned or loaded with the dead, a handful have been intercepted carrying live crews into Japanese waters. Last year a handful of fishermen were returned home along with the bodies of their crew mates.