Not all trash ends up at the dump. A river, sewer or beach can't catch everything the rain washes away, either. In fact, Earth's largest landfill isn't on land at all.

The Great Pacific Garbage Patch stretches across a swath of the North Pacific Ocean, forming a nebulous, floating junk yard on the high seas. It's the poster child for a worldwide problem: plastic that begins in human hands yet ends up in the ocean, often inside animals' stomachs or around their necks. This marine debris has sloshed into the public spotlight, thanks to growing media coverage as well as expeditions by scientists and explorers hoping to see plastic pollution in action.

What's it made of?

The Great Pacific Garbage Patch has sometimes been described as a "trash island," but that's a misconception, according to Holly Bamford, former director of the U.S. National Oceanic and Atmospheric Administration (NOAA) Marine Debris Program. If only things were that simple.

"We could just go out there and scoop up an island," Bamford said in 2009. "If it was one big mass, it would make our jobs a whole lot easier."

Instead, it's like a galaxy of garbage, populated by millions of smaller trash islands that may be hidden underwater or spread over many miles, as this NOAA video explains:

A study published in Nature, using data from vessel and aircraft surveys, found that 79,000 tons of plastic are floating in an area spanning 1.6 million square kilometers (about 618,000 square miles). Previously, researchers believed the area was four to 16 times smaller.

Recent ocean voyages have confirmed the garbage patch covers a huge area, and despite a lack of cohesion, it is relatively dense in places. Researchers have collected up to 750,000 pieces of microplastic from a single square kilometer, for example, and after conducting the first extensive aerial survey — a series of low-speed, low-altitude flights using multiple imaging techniques — the Ocean Cleanup foundation reported "more debris was recorded than what is expected to be found in the heart of the accumulation zone." (The Ocean Cleanup is the brainchild of Boyan Slat, a Dutch inventor who came up with the idea as a teenager. After years of development and testing, his first system launched in mid-2018.)

The most recent study in the journal Nature reveals just how tricky it will be to clean up. Researchers using submarine drones sampled the water off the California coast. The highest concentrations of microplastics were between 600 and 2,000 feet down, not at the surface. And that means trouble for the food chain.

"Even if you don’t care about the crabs and the larvaceans, they're the food of things you do care about – tuna, seabirds, whales and turtles all feed on them, or feed on things that feed on them," Anela Choy, a professor at the Scripps Institution of Oceanography at the University of California, San Diego and one of the paper’s authors, told USA Today.

While there's still much we don't understand about the garbage patch, we do know that most of it's made of plastic. And that's where the problems begin.

Unlike most other trash, plastic isn't usually biodegradable — that is, most of the microbes that break down other substances don't recognize plastic as food, leaving it to float there forever. Sunlight does eventually "photodegrade" the bonds in plastic polymers, reducing it to smaller and smaller pieces, but that just makes matters worse. The plastic still never goes away; it just becomes microscopic and may be eaten by tiny marine organisms, entering the food chain.

About 80 percent of debris in the Great Pacific Garbage Patch comes from land, much of which is plastic bags, bottles and various other consumer products. Free-floating fishing nets make up another 10 percent of all marine litter, or about 705,000 tons, according to U.N. estimates. The rest comes largely from recreational boaters, offshore oil rigs and large cargo ships, which drop about 10,000 steel shipping containers into the sea each year, full of things like hockey gloves, computer monitors, resin pellets and LEGOs. But despite such diversity — and plenty of metal, glass and rubber in the garbage patch — the majority of material is still plastic, since most everything else sinks or biodegrades before it gets there.

How is it formed?

NOAA Marine Debris Program

Earth has five or six major oceanic gyres — huge spirals of seawater formed by colliding currents — but one of the largest is the North Pacific Subtropical Gyre, filling most of the space between Japan and California. The upper part of this gyre, a few hundred miles north of Hawaii, is where warm water from the South Pacific crashes into cooler water from the north. Known as the North Pacific Subtropical Convergence Zone, this is also where the trash collects.

Bamford refers to the convergence zone as a "trash superhighway" because it ferries plastic rubbish along an east-west corridor that links two spinning eddies known as the Eastern Garbage Patch and the Western Garbage Patch.

Although the Pacific patch was the first known phenomenon of its kind, more have since been found in other oceans, including the Atlantic. And that shouldn't be surprising: According to a 2015 study, about 8 million metric tons of plastic now enter the ocean during a typical year, mostly coming from people who live within 50 kilometers (30 miles) of a coastline, but also from even farther inland.

"Eight million metric tons is the equivalent to finding five grocery bags full of plastic on every foot of coastline in the 192 countries we examined," study author and University of Georgia researcher Jenna Jambeck said at the time.

It may take several years for debris to reach a garbage patch, depending on its origin. Plastic can wash from interiors of continents to the sea via sewers, streams and rivers, or it might just wash away from the coast. Either way, it can be a six- or seven-year journey before it's in the garbage patch. On the other hand, fishing nets and shipping containers often fall right in with the rest of the trash. One of the most famous such debris spills came in 1992, when 28,000 rubber ducks fell overboard in the Pacific. To this day, the ducks still turn up on beaches around the world.

What's the problem?

Durable plastic debris threatens ocean health in several ways, including:

Entanglement: The growing number of abandoned plastic fishing nets is one of the greatest dangers from marine debris, Bamford says. The nets are notorious for entangling dolphins, seals, sea turtles and other animals in a phenomenon known as "ghost fishing," often drowning them. With more fishermen from developing countries now using plastic nets for their low cost and high durability, many lost or abandoned nets can continue fishing on their own for months or years. One of the most controversial types are bottom-set gill nets, which are buoyed by floats and anchored to the sea floor, sometimes stretching for thousands of feet.

Virtually any marine life can be endangered by plastic, but sea turtles seem especially susceptible. In addition to being entangled by fishing nets, they often swallow plastic bags, mistaking them for jellyfish, their main prey. They can also get caught up in a variety of other objects, such as this snapping turtle that grew up constricted by a plastic ring around its body.

Removing marine debris helps prevent these animals from ingesting debris, which will eventually kill them. The plastic offers no nutritional value and fills their stomachs, so they die of starvation.

Small surface debris: Plastic resin pellets are another common piece of marine debris; the tiny, industrial-use granules are shipped in bulk around the world, melted down at manufacturing sites and remolded into commercial plastics. Being so small and plentiful, they can easily get lost along the way, washing through the watershed with other plastics and into the sea. They tend to float there and eventually photodegrade, but that takes many years. In the meantime, they wreak havoc with sea birds like the short-tailed albatross.

Albatross parents leave their chicks on land in Pacific islands to go scour the ocean surface for food, namely protein-rich fish eggs. These are small dots bobbing just below the surface, and look unfortunately similar to resin pellets. Well-meaning albatrosses scoop up these pellets — along with other floating trash such as cigarette lighters — and return to feed the indigestible plastic to their chicks, which eventually die of starvation or ruptured organs. Decaying albatross chicks are frequently found with stomachs full of plastic debris — an image that's difficult to look at but impossible to ignore.

Photodegradation: As sunlight breaks down floating debris, the surface water thickens with suspended plastic bits. This is bad for a couple of reasons. First, Bamford says, is plastic's "inherent toxicity": It often contains colorants and chemicals like bisphenol-A, which studies have linked to various environmental and health problems, and these may leach out into the seawater. Plastic has also been shown to absorb pre-existing organic pollutants like PCBs from the surrounding seawater, which can enter the food chain — along with BPA and other inherent toxins — if the plastic bits are accidentally ingested by marine life.

What can we do?

The discoverer of the Pacific garbage patch, Capt. Charles Moore, once said a cleanup effort "would bankrupt any country and kill wildlife in the nets as it went."

"He makes a really good point there," Bamford says. "It's very difficult."

Still, NOAA conducts flyovers to study the garbage patch, and research teams have sailed there to collect debris and water samples. Scientists from the Scripps Institution of Oceanography held a press conference after returning from a three-week voyage in 2009, describing the amount of trash as "shocking." They found large and small items as well as a vast underwater haze of photodegraded plastic flakes, and continue to study how microplastic interacts with a marine environment.

Another study published in 2014 estimated that Earth's oceans now contain 5.25 trillion pieces of plastic overall, based on data from 24 trash-collecting voyages over a six-year period. That's a lot, but it still hasn't discouraged everyone from trying to clean it up — including the Ocean Cleanup foundation, whose research is part of a long-term remediation plan. (The initial Ocean Cleanup trial is ongoing — with some blips — but the foundation plans to send out more systems with full-scale deployment by 2020.)

Ultimately, even advocates of ambitious cleanup projects acknowledge that more plastic recycling — and wider use of biodegradable materials — is still the best hope for getting ocean plastic under control. Prevention is cheaper and easier, but as Bamford points out, old habits can be hard to break.

"We need to turn off the taps at the source. We need to educate people on the proper disposal of things that do not break up, like plastics," she says. "Opportunities for recycling have to increase, but, you know, some people buy three bottles of water a day. As a society, we have to get better at reusing what we buy."