Scientists have discovered high levels of extremely toxic chemicals in the most remote place on earth — the 36,000-foot-deep Mariana Trench in the western Pacific Ocean, according to new research published in the journal Natural Ecology and Evolution.

Marine biologists used fish traps and robotic submarines to collect crustaceans from the trench’s seafloor and then measured the level of persistent organic pollutants (POPs) in each specimen. They found that in the most polluted parts of the trench, crustaceans contained 50 times more POPs than crabs that live in one of China’s most polluted rivers. High levels of pollutants were also found in the Kermadec Trench, which is nearly 33,000 feet deep.

The most prevalent chemicals in the two deep-ocean trenches tested were PCBs, commonly used as coolant or lubricant in electrical devices, and PBDEs, used in flame-retardants. While neither chemical is manufactured today, they both take a very long time to break down naturally. This allows them to bioaccumulate as they move up the food chain. When large marine organisms such as whales die, their bodies sink to the ocean floor and are consumed by bottom-dwelling animals in deep regions such as the Mariana Trench.

“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 marine scientist at Newcastle University in the UK and lead author of the research, told The Guardian.