To paraphrase the immortal words of Diana Ross and the Supremes, ain’t no mountain high enough, ain’t no valley low enough to keep us from mucking it up. Case in point? The Pacific Ocean’s Mariana and Kermadec trenches, both tens of thousands of feet deep, remain two of the planet’s most inaccessible reaches. But even they are not immune to environmental damage from humans. Samples of amphipods—tiny, shrimp-like scavengers who call these dark, impenetrable depths home—have revealed “extraordinary levels” of persistent organic pollutants, according to new research. These included long-banned or restricted chemicals such as polychlorinated biphenyls and polybrominated diphenyl ethers, both of which are thought to cause neurological, immune, and reproductive issues, or even cancer.

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As published in the latest edition of Nature Ecology & Evolution, the findings offer a stark reminder of the extent of mankind’s impact. The Mariana and Kermadec trenches are 4,300 miles apart, yet toxic compounds were found “in all samples across all species at all depths in both trenches,” the researchers wrote.

Startlingly, the amphipods they sampled contained levels of contamination similar to those found in Japan’s Suruga Bay, a hotbed of industrial pollution. In the Mariana, the highest levels of PCBs were 50 times more concentrated than those found in crabs living in flooded plains fed by one of China’s most tainted rivers.

“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,” Alan Jamieson, a biologist from Newcastle University who led the study, said in a statement.

Related: James Cameron completes historic dive into deepest point on the planet

How the pollutants found their way into these extreme locales, which are characterized by immense pressure and a lack of light, is still a matter of conjecture, though the scientists have their suspicions. The chemicals may have found their way to the trenches through contaminated plastic waste and animal carcasses, which, like everything else in the ocean, eventually sink to the floor, where they’re devoured by resident fauna.

Because pollutants accumulate through the food chain, by the time they reach the deep ocean, they’re many times more concentrated than they were in shallower waters. The compounds could recirculate back to the surface as scavengers like amphipods fall prey to larger predators. (To quote another song, it’s “the circle of life.”)

“This research shows that far from being remote the deep ocean is highly connected to the surface waters. We’re very good at taking an ‘out of sight out of mind’ approach when it comes to the deep ocean but we can’t afford to be complacent,” Jamieson said. “The fact that we found such extraordinary levels of these pollutants in one of the most remote and inaccessible habitats on earth really brings home the long term, devastating impact that mankind is having on the planet.”

He added: “It’s not a great legacy that we’re leaving behind.”

+ Newcastle University