Incredible levels of toxic organic compounds have been discovered in the Mariana and the Kermadex Trench in the mid Pacific. The toxins were found in crustraceans collected between 8,000 and 10,000 metres below sea level at levels higher than the most polluted mines in Australia, Japan and the Philippines. Their use is banned in most developed nations.
The deepest trenches in the world are rarely explored and researched and the scientists were studying the overall ecology of the region, rather than specifically examining the impact of pollution.
The organic compounds concentrate in the oily tissue of animals and do not exist naturally. Separate research has revealed that it takes less than 100 days for pollutants to reach the trenches from the surface of the ocean.
http://www.nature.com/articles/s41559-016-0051
https://www.theguardian.com/environment/2017/feb/13/extraordinary-levels-of-toxic-pollution-found-in-10km-deep-mariana-trench
http://www.businessinsider.com/banned-chemicals-have-been-found-in-the-mariana-trench-2017-2?IR=T