Scientists have had a totally inadequate understanding.
Estimates of how much plastic has been dumped in the Atlantic Ocean have been "massively underestimated", experts say.
The mass of so-called invisible microplastics found in the upper waters of the Atlantic is around 12-21 million tonnes, new research suggests.
But the figure only represents three types of the most common types of plastic litter in a limited size range.
According to a study published in Nature Communications, it is comparable in magnitude to estimates of all plastic waste that has entered the Atlantic over the past 65 years - 17 million tonnes.
This suggests the supply of plastic to the ocean has been substantially underestimated, researchers say.
The study focused on polyethylene, polypropylene and polystyrene, which are commercially most prominent and also the most littered plastic types, according to the experts.
Lead author of the paper, Katsiaryna Pabortsava from the National Oceanography Centre (NOC), said: "Previously, we couldn't balance the mass of floating plastic we observed with the mass we thought had entered the ocean since 1950.
"This is because earlier studies hadn't been measuring the concentrations of 'invisible' microplastic particles beneath the ocean surface.
"Our research is the first to have done this across the entire Atlantic, from the UK to the Falklands."
Co-author Professor Richard Lampitt, also from the NOC, said: "If we assume that the concentration of microplastics we measured at around 200 metres deep is representative of that in the water mass to the seafloor below with an average depth of about 3,000 metres, then the Atlantic Ocean might hold about 200 million tonnes of plastic litter in this limited polymer type and size category.
"This is much more than is thought to have been supplied."
The researchers collected seawater samples during the 26th Atlantic Meridional Transect (AMT) expedition in September to November 2016.
They filtered large volumes of seawater at three selected depths in the top 200 metres and detected and identified plastic contaminants using state-of-the-art spectroscopic imaging techniques.
Professor Lampitt said: "In order to determine the dangers of plastic contamination to the environment and to humans we need good estimates of the amount and characteristics of this material, how it enters the ocean, how it degrades and then how toxic it is at these concentrations.
"This paper demonstrates that scientists have had a totally inadequate understanding of even the simplest of these factors, how much is there, and it would seem our estimates of how much is dumped into the ocean has been massively underestimated."
The work was funded by the EU H2020 AtlantOS programme and the NOC.
The AMT programme was supported by the UK Natural Environment Research Council's Climate Linked Atlantic Sector Science (Class) programme.
Nina Massey is the PA science correspondent.