PACIFIC – Researchers have returned from mapping and sampling a massive, swirling cluster of rubbish floating in the middle of the Pacific Ocean, as part of an ambitious mission to attempt to clean it up.

The scientists of the The Ocean Cleanup project, backed by volunteers in sailing boats, returned to San Francisco after mapping and sampling the Great Pacific Garbage Patch – a swirling mass of human-linked debris spanning hundreds of miles of open sea.

“I’ve studied plastic in all the world’s oceans, but never seen any area as polluted as the Great Pacific Garbage Patch,” said Dr Julia Reisser, the lead oceanographer with the project. “With every trawl we completed, thousands of miles from land, we just found lots and lots of plastic.”

The rubbish ranges from microscopic pieces of plastic to large chunks.

But the scientists said most of the trash they found is in medium to large-sized pieces, as opposed to tiny ones.

Working for about a month, the group collected samples as small as a grain of sand and as large as discarded fishing nets weighing more than 1000kg.

The debris, concentrated by circular, clockwise ocean currents within an oblong-shaped “convergence zone”, lies near the Hawaiian Islands, about midway between Japan and the US West Coast.

Volunteer crews on 30 boats from a 52-metre mother ship spent a month measuring the size and mapping the location of the mass of plastic waste that according to some estimates covers an area twice the size of Texas.

They mapped the area, using aerial balloons and trawling equipment to locate samples, said oceanographer Reisser.

“We did three types of surveys in 80 locations, and now we are working on completing an up-to-date estimate of the size of the patch, making a chart of hot spots and publishing our findings by mid-2016,” Reisser said.

“There were hundreds of times more plastics in these areas than there were living organisms,” she added.

The reconnaissance trip is the brainchild of Ocean Cleanup’s 21-year-old founder and CEO, Boyan Slat.

Slat, a Dutch inventor who gained attention as a teenager when he developed the floating boom system – which uses technology used for anchoring deep sea oil rigs – said the project would be situated in international waters, away from shipping lanes.

The project is backed financially by philanthropic business and crowdsourcing initiatives which brought in some $2.2 million, according to The Ocean Cleanup.

Although the samples collected during the expedition still have to be analysed, preliminary findings indicate a higher-than-expected volume of large plastic objects floating in the ocean.

This underscores the urgency of The Ocean Cleanup’s mission to clean it up, according to Slat: “The vast majority of the plastic in the garbage patch is currently locked up in large pieces of debris, but UV light is breaking it down into much more dangerous microplastics, vastly increasing the amount of microplastics over the next few decades if we don’t clean it up. It really is a ticking time bomb.”

The next phase, planned for 2016, is the deployment in Japanese coastal waters of a 2000-metre scale model of the group’s proposed debris collection system, which researchers believe could extend for 96 kilometres.

That system will contain floating stationary booms tethered to the ocean floor and linked in a V-shape intended to skim and concentrate surface plastics floating on top of ocean currents.

Slat said he became passionate about cleaning the oceans of plastic while diving in the Mediterranean sea five years ago.

“I was diving in Greece and realised that there were more plastic bags than fish,” he said, “and I wondered – why can’t we clean this up?”

After dropping out of university after six months, Slat dedicated his life to developing the technology the group will start testing next year.

He envisioned using long-distance floating barriers that will attach to the seabed and target swirling ocean currents full of waste to skim garbage from the surface while aquatic life and currents pass underneath.

After a 2012 Ted Talk about his idea was viewed more than two million times, Slat decided to launch a Kickstarter campaign and raised $2.27 million, helping to start his organisation.

Soon, his innovative solution got the attention of major philanthropists in Europe and Silicon Valley, including Salesforce.com chief executive Marc Benioff, who are helping pay for the data-gathering efforts and the technology’s development.

The cost of the reconnaissance expedition and the debris collection system was not disclosed. Critics say the system is too costly or unlikely to function as designed.