Many areas of the sea bed resemble a rubbish tip filled with bottles, plastic bags, discarded fishing nets and other examples of human litter, a survey has found.
Scientists took nearly 600 samples from across the Atlantic and Arctic Oceans, and the Mediterranean Sea, at depths ranging from 35 metres (115 feet) to 4.5 kilometres (2.8 miles).
Litter was found at each site, with plastic accounting for 41% and derelict fishing gear 34%. Glass and metal, wood, paper and cardboard, clothing, pottery, and a number of unidentified materials were also observed.
Christopher Pham, from the University of the Azores, said: "We found that plastic was the most common litter item found on the sea floor, while trash associated with fishing activities (discarded fishing lines and nets) was particularly common on seamounts, banks, mounds and ocean ridges. The most dense accumulations of litter were found in deep underwater canyons."
Co-researcher Dr Kerry Howell, from Plymouth University's Marine Institute, said: "This survey has shown that human litter is present in all marine habitats, from beaches to the most remote and deepest parts of the oceans. Most of the deep sea remains unexplored by humans and these are our first visits to many of these sites, but we were shocked to find that our rubbish has got there before us."
The scientists also found deposits of "clinker", residues of burned coal from steam ships from the late 18th century onwards. These closely followed modern shipping routes, showing that the main sea thoroughfares had not been altered in two centuries.
Plastic bottles and containers could be tracked from coastal and land sources along continental shelves and slopes until they eventually fell into deep canyons.
Dr Veerle Huvenne, another member of the team from the National Oceanography Centre, Southampton, said: "Submarine canyons form the main connection between shallow coastal waters and the deep sea.
"Canyons that are located close to major coastal towns and cities, such as the Lisbon Canyon offshore Portugal, or the Blanes Canyon offshore Barcelona, can funnel litter straight to water depths of 4,500 metres or more."
Dr Howell added: "The large quantity of litter reaching the deep ocean floor is a major issue worldwide. Our results highlight the extent of the problem and the need for action to prevent increasing accumulation of litter in marine environments."
The research appears in the online journal Public Library of Science ONE.