The numbers are shocking: Scientists have calculated that there are 5.25 trillion pieces of plastic garbage, for a total of 269,000 tons, floating in the ocean. Meanwhile, some deep sea regions are littered with tiny pieces of plastic known as microplastics.

Scientists have known for decades about the accumulating mass of ocean garbage and its deadly consequences for seabirds, fish and marine animals. Nonetheless, scientists still have a great deal to learn before they can fully understand the extent of the problem. For example, until last year almost nothing was known about the amount of plastic in remote regions of the Southern Hemisphere.

Until scientists learn more about where ocean plastic is, how densely it builds up and how it breaks down, they can't really calculate the damage it's causing. There are still big, unanswered questions: Do poisons in plastic seep into the marine environment as plastics break down? If so, how and in what amounts?

Scientists know a great deal about the damage to marine life caused by large pieces of plastic. However, the possible harm caused by tiny pieces known as microplastics is less clear. What effect do they have on fish that swallow them?

The greater the total amount of plastics, the greater the risks, said biologist Richard Thompson. He said we need to discover exactly where plastic is accumulating in order to measure its effects.

Scientists believe the 5.25 trillion figure accounts for only a fraction of the plastic that flows into the oceans every year. Where the rest of it remains a mystery.

Ocean plastic ranges from very large pieces to microscopically small fragments, Thompson said. "It is incredibly challenging to monitor."

By The Numbers

Ocean trash is counted in three ways: through beach surveys, computer models based on samples collected at sea, and estimates of the amount of trash entering the oceans. The most recent counts involved computer modeling based on samples taken at sea. The models may not account for all of the trash, scientists say. Nonetheless, the new numbers are helping scientists address some important questions.

The process of collecting and counting is detailed, time-consuming work. It took environmental scientist Marcus Eriksen more than four years to come up with his estimate that 269,000 tons of garbage float on the surface. During that time, Eriksen took 24 separate survey trips.

Erikson and his team are the ones who estimated the number of 5.25 trillion. In the course of his expeditions, Eriksen collected everything from plastic candy wrappers to giant balls of fish netting. One huge ball of netting contained 89 different kinds of net and line. It was all wrapped around a tiny, two-inch-high teddy bear.

Eriksen said his research helped answer some questions about the life cycle of ocean plastic. The plastic tends to collect in the world's five large gyres, which are large systems of spiraling currents. Then, as the plastic breaks down into fragments, it falls into deeper water. Currents carry it to remote parts of the globe.

"These fragments are anywhere on the planet at this point," he said. "We're finding them everywhere."

Sort Out The Rubbish

It's not too difficult to figure out why so much plastic ends up in the ocean. Around 33 percent of the plastic manufactured worldwide is used once, then thrown away. This estimate came from the Ocean Recovery Alliance, a Hong-Kong based organization. To makes things worse, 85 percent of the world's plastic is not recycled.

As serious as the problem is, it can be solved, scientist Peter Ryan said. We just have to "sort out what to do with our rubbish." Ryan has been tracking plastic garbage for 30 years. He began after a fellow scientist suggested he study seabirds that were eating floating plastic pellets, which at the time were commonly found in harbors and other waterways. Since then, improvements to shipping have reduced pellet spillage. "If you go to the beach today, you struggle to find one," he said.

However, progress of this sort has been overtaken by new problems, as microplastics have become more widespread. Emily Penn skippers the 72-foot Sea Dragon, which carries scientists on sea trash sampling surveys. She is still surprised and dismayed by the volume of trash that is routinely discovered.

"The thing that shocks me every time is the fact that the ocean looks like it is clear blue water," she said. Then, they "pull out that sock at the end of the net and find it's full of thousands of fragments of plastic."