At first, it seemed like good news: Measurements of the "garbage patch" in the Atlantic Ocean showed that the amount of plastic trash there hasn't increased over the past two decades.

Similar to the Great Pacific Garbage Patch, the North Atlantic garbage patch is somewhat like a region of plastic soup, although "soup implies you can see the vegetables," said study leader Kara Lavender Law, an oceanographer at the Sea Education Association (SEA) in Woods Hole, Massachusetts.

Instead, most of the Atlantic trash is in the form of tiny plastic bits—from bags and bottles blown off landfills or tossed into the sea—swirling in a still undefined region of open ocean hundreds of miles off the North American coast. (See "Plastic-Bag Bans Gaining Momentum Around the World.")

Law and colleagues recently analyzed data from the Atlantic garbage patch collected over the past 22 years and found that the concentration of stuff in the patch hasn't grown over time.

But even taking increased recycling rates into account, humans' plastic use over the past two decades has increased. So where has all the plastic gone?

It's possible some of the trash is just too small for researchers to catalog, study leader Law said: "Our net only captures pieces larger than [a third of a millimeter] in size, and it's certain that the plastic breaks down into pieces smaller than that."

Some of the fragments might have been eaten by sea creatures that mistook the plastic for plankton—tiny, free-floating marine plants and animals. The plastic pieces could also be sinking, weighed down by colonies of marine bacteria.

Another expedition to study the patch earlier this year—led by Anna Cummins and Marcus Eriksen of the Algalita Marine Research Foundation—had intended to stop and deploy a sediment sampler to test for plastic that might have sunk to the ocean floor. But that trip was cut short by bad weather, Cummins said.

Atlantic Plastic Patch Full of Mystery

Researchers can't tell the ages of individual plastic bits, since there are no chemical techniques for dating petroleum-based products, SEA's Law said. That makes it nearly impossible to tell if the seemingly stable amount of trash is actually due to turnover.

There's also currently no way to track the origins of the plastic: "We're not seeing Coke bottles stamped with 'Made in the U.S.A.,'" Law said. A computer model of ocean circulation suggests the fastest route to the patch begins at the U.S. East Coast.

Meanwhile, even though the SEA expedition traveled a thousand miles (1,609 kilometers) east of Bermuda, "we still can't find the eastern edge" of the patch, Law said.

The Algalita mission traveled much farther east, all the way to the Azores islands off Portugal (map), the foundation's Eriksen said, and "we had plastics in our samples from Bermuda to the Azores."

Overall, the results tell "a depressing story, to be honest," SEA's Law added. "You're like, Great, [the patch] is not increasing! But I'm sure it is."