Plastic never goes away.
Plastic is a substance the earth cannot digest. It is a material made to last forever, yet 33 percent of all plastic – water bottles, bags and straws – are used just once and thrown away.
“Plastics will degrade into small pieces until you can’t see them anymore (so small you’d need a microscope or better!). Because the ocean is a cold, dark place, this process happens slower in water than on land. But, do plastics fully “go away?” Full degradation into carbon dioxide, water, and inorganic molecules is called mineralization. Most commonly used plastics do not mineralize (or go away) in the ocean and instead break down into smaller and smaller pieces. We call these pieces “microplastics” if they are less than 5mm long. Bio-based and truly biodegradable plastics break down in a compost pile or landfill, but are generally not designed to degrade as quickly in the ocean.”
*Source: National Oceanographic And Atmospheric Admnistration, US Department of Commerce “What We Know About: Plastic Marine Debris”