Due to the Earth's rotation, we have wind flows and the weather. There is something called the Coriolis Effect that dictates which way the winds blow, and when. This wind also affects ocean currents. Almost 75% of the globe is covered by water and there are very specific currents that flow all year round, from one region to another. The colder water from the polar region flows towards the warmer Equatorial region, warms up and flows back to where it came from ... in a never ending cycle. These ocean current cause the ocean waters to churn over a large geographical area creating what are called gyres. These are large circulating bodies of water going round their own axis as the larger water body around them flow all around the globe in a continuing cycle.
There are many such gyres in our oceans, but the five biggest and main ones are the Indian Ocean Gyre, the North Atlantic Gyre, The South Atlantic Gyre, the North Pacific Gyre and the South Pacific Gyre.
The problem that these Gyres are facing over the last few decades is that they are becoming garbage islands in the oceans. Due to the tides, currents, winds, etc, any debris that is not disposed off properly on shore, finds its way into the oceans, flows along with the currents and gets trapped in one or the other gyres. When the debris is plastic the problem takes on significant proportions. Plastic does not biodegrade and the Earth cannot digest it. With time and with exposure to the Sun, plastic becomes brittle and breaks down into smaller and smaller particles, finally becoming microscopic. While still visible they are mistaken for food by marine life, who then consciously consume it. Their stomachs cannot digest the plastic till they cannot eat anymore and ultimately die of starvation ... with a stomach full of plastic
Fish too ingest plastic particles and when they end up on our plates, we humans end up ingesting plastic too, leading to long term health concerns.
The plastic garbage in the Gyres are relatively low density and the garbage is not visible by satellite imagery. The particles are often missed by boaters or swimmers due to their small size. Microscopic particles are eaten by marine life and the larger pieces cause immense damage to them too. Turtles love jelly fish and they end up consuming floating plastic bags that look like jelly fish. Fish get caught in nets and fishing line.
The largest of the garbage patches is the Great Pacific Garbage Patch, created by the swirling waters of the North Pacific Gyre.
This is largely in what is called the Horse latitudes and the debris swirls around in the currents, gradually moving towards the centre, and gets trapped there. Origin of the debris is fro discarded fishing gear and improper on-shore management of trash. A carelessly disposed off drinking straw at a restaurant in San Francisco will add to the volume of the Great Pacific Garbage Patch in about six years, while the same straw from Kolkata will join his forgotten buddies in about a year.
Estimates about the size of the Great Pacific Garbage Patch range from the size of Texas (about 700,000 sq km) to the size of Russia (about 15 million sq km).
Marine debris is region agnostic. They do not require permits or passports to travel to far away lands. Midway Island is a six square kilometre rock in the middle of the Pacific Ocean, 5,000 miles or more from the closest continent. The Island receives more than 20 tonnes of plastic every year, about 50 kg every week..
If you feel happy that you have discarded your plastic trash in the relevant bin, be aware that much of it is still going to end up in the ocean and add to marine debris.