You'd think that seven miles beneath the Pacific Ocean's surface would be a place so remote that human activity couldn't screw it up; sadly, that's not the case.  Some of man's worst pollutants have made it all the way down to the Ocean's deepest waters.

The temperature in the Mariana Trench is near freezing, and the pressure would crush a human.  So biologist Alan Jamieson of Newcastle University in England and his team sent a robotic submarine down into the trench, taking images and recovering samples of water and the small crustaceans that can survive in this niche.

The crustaceans, called amphipods, were contaminated with polychlorinated biphenyls, known as PCBs.  The toxic chemicals was used for decades in industry, but were believed to be carcinogens and were thus banned in Australia and the US in the 1970s. 

"Every sample we had," said Professor Jamieson, "had contaminants in it at very high or extraordinarily high levels."  After comparing the Mariana Amphipods to crabs living in waters fed by one of China's most polluted rivers - as well as amphipods from other parts of the world - Jamieson and his team were shocked:  "What we were finding in the deepest place in the world were (levels) hugely higher, 50 times in some cases," he says.

The research is published in the journal Nature Ecology and Evolution.  Jamieson believes that chemical concentrations in the Mariana Trench is simply the work of gravity.  Pollutants might latch to plastic trash in the ocean that eventually floats to the bottom.  Marine life absorb the pollutants as well, and  the dead animals and fish poop makes its way down as well. 

"Once it gets deeper and deeper and deeper, there's nowhere else for it to go, because there's no mechanism to put it back to the surface again," Jamieson explains.