Humans, being filthy creatures, create a lot of garbage. Despite our best efforts to dispose of it properly, sometimes that garbage finds its way to unintended destinations, like the world’s oceans. Researchers at the University of Western Australia, however, have made a peculiar finding: There isn’t nearly as much garbage in the ocean as there ought to be, particularly plastic garbage. The study suggests that smaller plastic particles are being held in an unknown sink, somewhere.

As plastic production technologies in the mid-1900s made mass production more common, concentrations of plastic debris in the world’s oceans increased right alongside production volume. Curiously, concentrations leveled out in the 1980s, despite continued production and disposal. For the study, the authors collected 3,070 ocean surface samples at 141 sites around the world and found that 88 per cent of the samples contained plastic debris of varying sizes with highest plastics concentrations in the five subtropical ocean gyres.

What they found was that the smallest particles (smaller than one millimeter) appeared in lower concentrations than their forecasts predicted. The discrepancy reduced the total volume estimates for plastic pollution by tens of thousands of tons, but the debris doesn’t just magically disappear. Scientists aren’t sure whether ocean life or another system altogether is responsible for the removal, but it has to be somewhere.

“This sink may involve a combination of fast nano-fragmentation of the microplastic into particles of microns or smaller, their transference to the ocean interior by food webs and ballasting processes, and processes yet to be discovered,” the authors wrote in the study.

They go on to say that resolving the fate of the missing particles is “of fundamental importance” to further determine the total impact of plastic pollution on oceanic ecosystems.