I found out where some of it is:

There’s an area known as the North Pacific subtropical gyre which contains a mass of floating plastic trash twice the size of Texas, see: http://www.bestlifeonline.com/cms/publish/travel-leisure/Our_oceans_are_turning_into_plastic_are_we.shtml.

This is where the bags and bottles accumulate under the influence of ocean currents and winds.

According to Charles Moore, who has been carrying out studies of this area called “The Garbage Patch”, the sea there contains, by weight, six times as much plastic as it does plankton.

Moore says that the North Pacific gyre is only one of five such high-pressure zones in the oceans. There are similar areas in the South Pacific, the North and South Atlantic, and the Indian Ocean, each with its own version of the Garbage Patch. Together, these areas cover 25 % of the planet.

But that’s only the floating trash. According to Greenpeace, about 70 percent of the trash, around 6.5 million tons of plastic rubbish, lies out of sight on the seabed. The great bulk of this waste is made up of discarded plastic containers and bags.

This growing mass of stuff, besides being ugly, is of concern for several reasons. The more well-known problem is the havoc wreaked on wildlife. Sea turtles swallow the bags, mistaking them for their favorite jelly fish. Nearly one million seabirds and 100 thousand marine mammals may die from entanglement or ingestion of floatable debris each year."

(For the whole article see: http://earthdaynetwork.spaces.live.com/mmm2007-10-25_18.59/blog/cns!FE94E768F855775C!665.entry )