Old-fashioned metal boxes that have been dragged around the ocean since 1931 have accidentally created a record of the history of ocean plastic, BBC reports.
The devices - known as continuous plankton recorders (CPRs) - first ensnared a plastic bag off the coast of Ireland in 1965.
This, researchers say, could be the first marine plastic litter found.
The CPR record also revealed how much more plastic has been found in the ocean in recent decades.
By fishing for plankton for all those decades - a key species that indicates the productivity of the ocean and so of particular interest for monitoring the health of fisheries - the machines also produced a history of plastic litter.
Lead researcher Dr Clare Ostle, from Plymouth's Marine Biological Association, explained that the "fleet" of CPRs were designed and built to be towed behind ships. They capture samples of plankton from the water column - trapping them on a mesh inside.