We’ve all heard of those patches of garbage floating in the middles of the oceans, but what about the stuff that sinks? To get an idea how trashed the deep sea is, researchers at the Monterey Bay Aquarium Research Institute (MBARI) did something pretty clever: They dug into 18,000 hours of deep sea videos captured by remotely operated vehicles over the last 22 years.

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“We were inspired by a fisheries study off Southern California that looked at seafloor trash down to 365 meters, said Kyra Schlining, lead author of a study which appears in the journal Deep-Sea Research I. “We were able to continue this search in deeper water — down to 4,000 meters. Our study also covered a longer time period, and included more in-situ observations of deep-sea debris than any previous study I’m aware of.”

Over the past two decades the video technicians at MBARI recorded every object and animal that appeared in the videos. So it was a matter of mining all that information to see which videos showed trash, and then compiling the trash sightings, which came to more than 1,500 at dive sites ranging from Monterey Bay, Vancouver Island, and the Gulf of California all the way to the Hawaiian Islands. In Monterey Submarine Canyon alone they found more than 1,150 piece of trash.

The most common trash was plastic bags, aluminum cans and fishing equipment. A full third of the garbage was plastic and half of that plastic was bags, which can smother or choke some marine life. Metal junk came in second with aluminum, steel or tin cans making up the majority of that. The rest of the debris was rope, glass, fishing gear, paper and textiles.

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They also discovered that gravity works just the same at sea as on land: Most of the trash was seen below 2,000 meters (6,500 feet). At those depths, the water is pitch black, near freezing and there isn’t much oxygen, so biodegrading the debris could take decades.

“I’m sure that there’s a lot more debris in the canyon that we’re not seeing,” said Schlining. “A lot of it gets buried by underwater landslides and sediment movement. Some of it may also be carried into deeper water, farther down the canyon.

She also expressed frustration that the bulk of the trash are recyclables. Ultimately, wrote Schlining and her colleagues, the best approach to reducing deep sea trash is by increasing public awareness.

Photo: A tire sits on a ledge 868 meters (2,850 feet) below the ocean surface in Monterey Canyon. Image: ©2009 MBARI