In a surprise finding, scientists say the amount of pollutants in the Pacific Ocean’s Mariana Trench—one of the most remote locations on Earth—is so high that it outpaces the amounts found in a heavily polluted Chinese river.

Crustaceans that live in the trench, which extends 36,000 feet (11,000 meters) below sea level, were captured by a robotic submarine. In the journal Nature, Ecology & Evolution, scientists reported that they found 50 times more pollutants in those crustaceans than in crabs from paddy fields fed by the Liaohe River, one of the most polluted rivers in China.

The chemicals discovered by scientists include two persistent organic pollutants, or POPs, produced between the 1930s and 1970s. The study reports that about 1.3 million tonnes of these chemicals were produced globally in that timeframe, and some of them were released into the environment because of industrial accidents and discharges, landfill leaks, or incomplete incineration. Since POPs don’t break down easily, they linger in the environment long after they arrive in it.

“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 of Newcastle University in the UK, who led the research, told the Guardian. “The fact that we found such extraordinary levels of these pollutants really brings home the long-term, devastating impact that mankind is having on the planet.”

The POPs that found their way into the trench were most likely from industrialized regions in the Northwest Pacific. They sank through the water column (from being trapped underneath sinking substances like degrading plastic and debris from the nearby Great Pacific Garbage Patch) and collected in the trench, where they have stayed because the trench doesn’t get a lot of water flowing into it to disburse the chemicals.

It’s possible that POPs built up fast in the trench. Sinking radiation from the 2011 Fukushima Daichi nuclear disaster allowed scientists to figure out how fast material from the surface sinks: the amount of time it takes is between three and six months.