Deep beneath the ocean, in some of the earth’s deepest trenches, chemical pollution has been found in shocking levels, including in the Mariana Trench. These areas were believed to be untouched by humans, but scientists have discovered pollution on par with some of the most polluted waterways on the planet. 
PCBs were used to make electrical equipment in the U.S. before being banned in the 1970s over health and environmental concerns. PBDEs are used as flame retardants. The U.S. has restricted the manufacture and importation of most PBDEs, though 1 common type of PBDE is still permitted.
Both PCBs and PBDEs have been linked to neurological, immune, and reproductive problems in humans, as well as cancer.
Despite the ban on PCBs and restrictions on PBDEs, the pollutants are pervasive because of their once-widespread use, and can be detected in marine animals. The substances can linger for many years and bind to other particles in the water that can carry them throughout the ocean.
The chemicals can also bioaccumulate, meaning they can build up in marine organisms over time. For instance, a 2016 study found that PCBs and PBDEs and other organic pollutants are prevalent in fish worldwide.
The researchers sent a remotely-controlled vehicle deep inside the Mariana trench, where they snapped photos, collected water samples, and baited animals. The discovery of the pollutants came as a shock because the team wasn’t looking for them and didn’t expect to find them in a region of the ocean that was thought to be pure.
The submarine emerged carrying crustaceans that were inundated with toxin levels up to 50 times greater than crustaceans that live in some of the most polluted rivers in China. 
Said Alan Jamieson, who led the team of researchers:
“You think we’re at the Mount Everest of the ocean, the very deepest point, and the levels were coming out orders of magnitude higher than places you would expect for it to be really high.”
“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 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 findings suggest that persistent organic pollutants (POPs) make their way into the deepest recesses of the ocean as dead animals and bits of plastic accumulate in fat and are therefore concentrated in creatures up the food chain.
And since the Mariana trench is so far from industrial sources, it shows that POPs travel long distances and remain intact over many years.
Jamieson said of the pollution:
“When it gets down into the trenches, there is nowhere else for it to go. The surprise was just how high the levels were – the contamination in the animals was sky high.”
The researchers documented their findings in Nature Ecology & Evolution.
Beginning in the 1930s, Monsanto was a major manufacturer of PCBs, and that legacy has stuck with them over the decades, to the company’s dismay. Monsanto-made PCBs have had a “severe impact” on European killer whales and bottlenose dolphins. The problem is so severe that researchers have warned that the chemicals are driving the animals out of existence.
The Guardian reported:
“The UK’s last pod of killer whales is doomed to extinction, with new research revealing western European waters as a global hotspot for the lingering legacy of toxic PCB pollution.”
In December 2016, Washington State announced a lawsuit against Monsanto alleging that the company’s PCBs have polluted “every waterway in the state.” The state wants the biotech giant to pay for the costs associated with cleaning up the toxins.
 The Guardian