the article doesn't say it, but this all suggests someone probably had a bunch of old crap in a tank and dumped in the ocean. Maybe it was a giant ocean liner amount of it. Right? Why does no one want to just say that outright?
Alternately, shipping containers are regularly lost from container ships.
The article doesn't say if this was closer to raw material or waste.
Mostly because the article is about a chemist focussed on identifying WTF it is and less so an expert in shipping/unreported damage/clandestine dumping.