Even the deepest most remote parts of the ocean can’t escape our foolishness.

Scientist have found a large amount of banned chemicals in the fatty tissue of crustaceans that live in the Pacific Ocean’s Mariana and Kermadec trenches. 

Though the locations are 10 kilometres (six miles) deep and far from land, researchers from Newcastle University in the U.K. identified traces of prohibited chemicals once used as fire retardants in the tiny amphipods.

Using deep sea landers to recover the samples, one of the chemicals scientists found were polychlorinated biphenyls (PCBs). PCBs were banned in the U.S. in 1979 and no longer imported into Australia from 1975, amid environmental and health fears. 

The report claims that around 1.3 million tonnes of PCB was created between the 1930s and the 1970s, with about 65 percent of that now contained in landfills and electrical equipment. A further 35 percent may be stuck on the coast and in the ocean.

“We still think of the deep ocean as being this remote and pristine realm, safe from human impact, but our research shows that, sadly, this could not be further from the truth,” the report’s lead author Alan Jamieson said in a statement.

He found the shrimp-like creatures contained chemical levels comparable to some of our most degraded environments: “In fact, the amphipods we sampled contained levels of contamination similar to that found in Suruga Bay, one of the most polluted industrial zones of the northwest Pacific.”

Published in the journal Nature Ecology & Evolution, the report points out that our oceans often act as a “sink” for chemicals released by insecure landfills and industrial accidents.

It’s important to remember these are human-made chemicals that do not occur naturally said Katherine Dafforn, a senior research associate at the University of New South Wales, who researches the effect of toxic chemicals on the marine environment. She was not involved in the report.

Dafforn described the study as “scary,” saying “I would have thought of these trenches as far removed from any of the problems we’re creating on land,” she added. “I’ve always thought of them very far removed from human activities.”

One reason why such chemicals are the focus of marine research is they don’t break down quickly in any environment, persisting over decades while accumulating in living organisms in high concentrations.

“This impacts on the hormonal endocrine system, which can effect development,” Dafforn explained. This could result in problems with growth and the appearance of tumours on fish, for example, as well as potentially damaging the reproductive system.

The report points out that our oceans often act as a “sink” for chemicals.

How these chemicals are falling into the trenches is still unknown, although the researchers speculate they could be delivered in part by falling carrion including dead fish and organic matter as well as falling plastic debris. 

“These trenches are very deep — once something is in them, it’s very hard for it to come back out,” Dafforn pointed out. PCBs have a knack for finding their way into isolated environments, including the Arctic.

In Dafforn’s view, there are still many “black boxes” out there when it comes to the impact of man-made chemicals on the environment. Of particular concern to her are microplastics — tiny bits of plastic that occur thanks to the breakdown of plastics bags and bottles, as well as from cosmetic products like microbeads.

In other words, we’re far from done doing damage.