Humans have visited the deepest part of our planet, the Mariana Trench, only twice: once in 1960, and again in 2012.
Yet the trench — a nearly 7-mile deep rift in the Pacific — appears to be pretty polluted. Some animals there have contamination levels similar to what you'd see in highly polluted industrial areas. They also contain toxins banned in the 1970s.
The pollutants seem to concentrate thanks to gravity and the food chain. As animals who were contaminated near the surface die and sink to the bottom, the toxins build up in the ecosystem.
The toxins are called Persistent Organic Pollutants. They can cause birth defects and developmental problems in both animals and people. The most infamous example of this type of toxin is probably the pesticide DDT.