By Judith Burns
Science and environment reporter, BBC News
Abandoned fishing gear was among the debris found by scientists
Scientists have confirmed that there are millions of tonnes of plastic floating in an area of ocean known as the North Pacific Gyre.
The first of two ships on a voyage to study plastic pollution there has recently returned to port.
Scientists on board say they found increasing amounts of plastic of all sizes as they travelled into the gyre.
They plan to analyse the effects of the waste on marine life and will propose methods to clear it up.
The North Pacific Gyre is a slow-moving clockwise vortex where four major ocean currents meet. Little lives there besides phytoplankton.
Larger than Texas
However the currents have carried millions of tonnes of rubbish into the centre of the gyre, which now covers an area estimated to be larger than the US state of Texas.
The two ships from Project Kaisei set off for the gyre from San Francisco more than three weeks ago.
The research vessel New Horizon from the Scripps Institution of Oceanography arrived back earlier this week. The second, the tall ship Kaisei, will be back on Monday.
The scientists conducted more than 50 surface debris trawls in 17 sites, studying and detailing debris and invasive species.
This collection of plastic was pulled from the ocean in just one hour
They were shocked by the amount of plastic they found.
Project director Doug Woodring said: "One thousand miles from shore with no sign of human life for days, yet our human footprint is now apparent in even one of the most remote places on the planet."
Mary Crowley, Project Kaisei co-founder, said: "More than 30 years ago, on my first trip to the North Pacific Gyre I found a few glass ball fishing floats, one net and there were, in four days, perhaps two pieces of floating plastic.
"Returning now with Project Kaisei .. the marine debris situation shows a startling change in this same area. In 30 minutes one easily can count up to 400 pieces of plastic on the sea's surface."
Tiny pieces of plastic film can be ingested by sea jellies
The team found a variety of invertebrates living in the debris, including crabs, sea anemones, barnacles, sponges and algae, sparking fears that the plastic may aid the spread of invasive species.
The researchers now plan to carry out extensive laboratory testing and analysis on the pieces of plastic they have collected, looking for toxins such as DDT and PCB.
The ultimate aim of the project is to develop sound scientific sampling of marine debris, to assess prototype technologies for removing the waste and to gain insight into how future clean-up programmes might work.