Plastic doesn't biodegrade easily. Most plastic take thousands, if not tens of thousands, of years to chemically break down, and we've made a lot of it since the material was discovered in 1855 by Alexander Parkes. Over the past 50 years, we've increased the adoption of plastic in our everyday lives and as it currently stands, it's nearly impossible to live without it. Today we use plastic in everything — from food packaging to sports equipment. Most of that plastic gets thrown away after being used, and when plastic doesn't make it to the trash can, it often ends up in waterways that carry it to the sea.
In the ocean, the plastic gets taken by the currents, swarming in a few distinct zones spread around the planet. The currents swirl in broad circular patterns, leaving the plastic to swirl around in the middle. These zones, also known as gyres, are enormous — the largest in the Pacific Ocean is estimated to be larger than the continental United States. In the soupy plastic stew that makes up the waters within the gyres, the plastic literally outnumbers the plankton. The sun and sea breakdown the plastic into smaller pieces of plastic which eventually can be smaller than grains of sand. Sea animals mistake the plastic for food and tens, if not hundreds, of thousands have died with stomachs full of it.
There's not much we can do to clean up the gyres with current technology. The best we can do is to use plastics better and try to stop adding to the problem.