 Marine debris, primarily made up of plastic, is a global issue that negatively affects wildlife, tourism, and shipping. Despite the durability of plastic and the exponential increase in its production, monitoring data shows limited evidence of corresponding increases in concentrations of plastic in marine habitats. There is a significant portion of manufactured plastic that remains unaccounted for in surveys tracking the fate of environmental plastics. Even the discovery of widespread accumulation of microscopic fragments, microplastics in oceanic gyres and shallow water sediments does not fully explain the missing fraction. In this study, we demonstrate that deep sea sediments are a lightly sink for microplastics. Microplastic, in the form of fibres, was up to four orders of magnitude more abundant per unit volume in deep sea sediments from the Atlantic Ocean, Mediterranean Sea, and Indian Ocean than in contaminated sea surface waters. Our findings provide evidence for a large and previously unknown repository of microplastics. The dominance of microfibres suggests that a previously under-reported and unsampled plastic fraction may be responsible. Given the vastness of the deep sea and the prevalence of microplastics at all sites we investigated, the deep sea floor appears to be the answer to the question, where is all the plastic? This article was authored by Lucy C. Woodall, Anna Sanchez-Widdell, Mikwell Canals, and others.