1. What Researchers Found at the Deepest Spot on Record
Japan's marine research agency JAMSTEC has filmed and photographed the deep seafloor on its dives and kept the records in a collection called the Deep-sea Debris Database. A team led by Sanae Chiba went through 30 years of these records and counted the man-made objects in them. In 5,010 dives they counted 3,425 items [1].
More than a third of the items were large pieces of plastic, and 89% of those were single-use products such as bags. Below 6,000 meters, plastic made up 52% of the trash and single-use products 92% of that plastic. The deepest item was a plastic bag at 10,898 meters in the Mariana Trench [1].
Two cautions. Those percentages describe the trash that was counted, not all the plastic in the ocean. And "deepest" means deepest in this database. It does not prove no deeper bag exists.
2. Why Plastic That Floats Can Still Sink
Many plastics are lighter than water, so a bag should stay at the surface. Floating plastic does not stay clean, though. Tiny organisms and algae form a slimy film on it, a process called biofouling. As the film grows, the object gets heavier overall and can start to sink [2].
A computer model from a Dutch research team explored what this means for tiny pieces. It predicted that, depending on size and density, a particle may keep floating, sink all the way to the seafloor, or bob up and down as its coating grows and wears away. Some very small pieces might never reach the bottom [2]. This is a prediction from a model, not a measurement made at sea.
3. Where on the Seafloor the Plastic Collects
The seafloor is not evenly littered. In a part of the western North Pacific, at depths from 1,092 to 5,977 meters, the team estimated 17 to 335 plastic items per square kilometer. Plastic turned up even in the open ocean more than 1,000 kilometers from land, which suggests that trash from human activity on land has reached far out [1]. That estimate comes from some of the dives, so it is not an average for the whole ocean.
For tiny pieces, a research team led by Ian Kane at the University of Manchester studied the seafloor of the Tyrrhenian Sea, part of the Mediterranean Sea. They found that slow, steady currents near the bottom act like conveyor belts. These currents sweep fibers and fragments into huge drifts of sediment, where they are packed together. In the richest spot, there were up to 1.9 million pieces in a thin layer covering just one square meter, the highest level recorded on the seafloor when the study came out in 2020 [3]. That number is the peak of one hotspot in one sea, not a typical seafloor.
The researchers also reported that most of this seafloor microplastic is made of fibers from textiles and clothing. They add that the currents carry oxygen and food, so these hotspots can be home to deep-sea life [3].
4. What Happens When Deep-Sea Animals Meet Plastic
Does anything actually eat it? A team led by Alan Jamieson caught small shrimp-like animals called amphipods in six deep trenches around the Pacific, at depths of 7,000 to 10,890 meters. In 65 of 90 animals (over 72%), the hindgut held at least one tiny particle. The mean count per animal ranged from 0.9 in the New Hebrides Trench to 3.3 in the Mariana Trench [4].
Not every particle was plastic. When the team analyzed a sample of them, they found nylon, polyethylene and polyvinyl chloride, but also rayon and lyocell (semi-synthetic fibers made from plant material) and ramie, a natural fiber [4]. So the finding is that man-made particles are within reach of animals in the deepest places on Earth, not that every animal is full of plastic.
In the video record, deep-sea organisms appeared in 17% of the images of plastic debris. In one case, a plastic bag was tangled around a community of animals living at a cold seep, where chemicals leak from the seafloor [1]. Appearing in the same picture is not the same as being harmed by the plastic.
5. Can Anything Break Plastic Down in the Deep Sea?
Ordinary petroleum-based plastics are reported to resist breaking down in nature, lasting decades or longer. Some plastics, though, are designed to be eaten by microbes. A group led by Tadahisa Iwata at the University of Tokyo, with Japanese deep-sea researchers, left 18 kinds of biodegradable plastic and four kinds of petroleum-based plastic at five sites on the seafloor near Japan, from 757 to 5,552 meters deep. After 3 to 14 months, they brought the samples back up and examined them [5].
Most of the biodegradable samples were covered in microbes and showed signs of being eaten away. The petroleum-based plastics, including polypropylene and the kind used for water bottles, showed no sign of attack, and neither did polylactic acid, a popular biodegradable plastic. The process was slow. At 1,000 meters it ran at one-fifth to one-tenth of the speed on a shore, and at 5,000 meters about one-twentieth. The researchers suggest pressure, temperature and fewer microbes may be reasons [5]. The group estimates that a bag made of one of these materials, PHA, would take about 3 weeks to 2 months to break down at 1,000 meters [5].
So the deep sea can break down some plastics, slowly, but that does not apply to the ordinary bags and bottles in the footage.
6. A Test You Can Try at Home: Which Plastics Float or Sink? And Where to Read More
Fill a tub or sink with water. Choose a few clean plastic items that fit easily, such as a bottle cap, a chip bag, and the corner of a food tray. Drop them in and watch which float and which sink. Ask an adult to help if you need scissors. Recycle or throw the items away as usual.
This only shows the starting point. In the sea, the slimy film described above and the movement of water also matter, so the tub cannot tell you what happens at 10,000 meters [2].
One big question remains open: how much plastic reaches the deep sea in total. The sources here do not settle it. For a first look, the Manchester press release [3] is a readable place to start, and the abstracts of the research papers are free to read [1] [4].