Currents in the North Pacific Ocean move rubbish into calm seas, where it accumulates to form the Plastic Vortex. The North Pacific Subtropical Gyre is an oval-shaped vortex of air and water moving clockwise between the equator and 50?N latitude. Trash has been sucked into the centre of the vortex over the years.

The high air pressure and calm waters facilitate rubbish accumulation on both sides of Hawaii - known as the Eastern and Western Pacific Garbage Patch. Most of the rubbish is trapped, except for small amounts that leak into the Hawaiian archipelago during storms.

Fishermen and sailors avoid crossing the gyre area, as there are fewer fish and only light winds. That could be why the litter vortex was not discovered until the 1990s. Seventy per cent of the litter has sunk to the seabed, while the remainder floats on the water and is two to three metres deep, Doug Woodring from Project Kaisei says. The vortex is a 'soup' rather than solid, as a lot of waste has degenerated into pebble-size particles. The area's low density hides it from satellites. Of the five ocean gyres, the biggest is in the North Pacific. Asia and North America, which sandwich the North Pacific, are more populated and generate more waste, he says.