NOT far off the coast of Guam lies the deepest point on Earth's surface, the Mariana trench. Its floor is 10,994 metres below sea level. If Mount Everest were flipped upside down into it, there would still be more than 2km of clear water between the mountain's base and the top of the ocean. Such isolation has led many to assume that it and similar seabed trenches will be among the few remaining pristine places on the planet. However, a study led by Alan Jamieson of Newcastle University in England, has shown that nothing could be further from the truth. As Dr Jamieson and his colleagues report this week in Nature Ecology and Evolution, trenches actually are loaded with pollutants.

Despite the cold, the darkness and the high pressure, ocean trenches are home to ecosystems similar in many ways to those found on other parts of the planet. In one important respect, though, they are different. This is where the energy that powers them comes from. In most ecosystems, sunlight fuels the growth of plants which are then consumed by animals. In a few shallower parts of the ocean, hydrothermal vents provide energy-rich chemicals that form the basis of local food chains. No vents are known to exist below 5,000 metres, though, and no sunlight penetrates a trench. The organisms found in them thus depend entirely on dead organic material raining down upon them from far above.

Since these nutrients, having once flowed into a trench, never make their way out again, Dr Jamieson found the notion that trenches have somehow remaining untouched by human activities questionable. He suspected that long-lived pollutants such as polychlorinated biphenyls (which were once used widely in electrical equipment) and polybrominated diphenyl ethers (employed in the past as flame retardants) might have made their way into the bodies of organisms living in trenches.

To test this idea out, he and his colleagues sent an unmanned lander to the bottom of the Mariana trench and also to the bottom of the Kermadec trench, near New Zealand. This lander fell to the seabed and spent between eight and 12 hours there, capturing amphipods (a type of crustacean, pictured) using funnel traps baited with mackerel. At the end of its mission it jettisoned some ballast and floated back to the surface with its prey.

In total, the lander was able to collect specimens from ten sites in the two trenches. The shallowest site sampled was 7,227 metres down in the Kermadec trench. The deepest was 10,250 metres in the Mariana. When the team looked for pollutants in the captured amphipods, they found that polybrominated diphenyl ethers were indeed present, but at moderate concentrations. Levels of polychlorinated biphenyls, however, were almost off the scale.

In animals collected from clean coastal environments, polychlorinated-biphenyl levels do not normally exceed one nanogram (billionth of a gram) per gram of tissue. In grossly polluted areas, like the Liao River in China, that level may rise a bit above 100 nanograms. In the Mariana trench, Dr Jamieson found, amphipods dwelling at 10,250 metres yielded 495 nanograms per gram of the pollutant. Those 8,942 metres down yielded 800 nanograms. And at 7,841 metres he and his colleagues discovered the staggering level of 1,900 nanograms per gram of amphipod tissue analysed. Values from the Kermadec trench were more modest, but still pretty high—ranging from 50 nanograms to 250 nanograms per gram.

Precisely why the Mariana trench has such elevated levels of polychlorinated biphenyls remains unclear. Dr Jamieson suspects it has to do with the trench's proximity to the North Pacific Subtropical Gyre, a whirlpool hundreds of kilometres across that has amassed enormous quantities of plastics over the years, and which has the potential to send the pollutants that bind to those plastics deep into the ocean as the plastics degrade and descend.

What consequences all this has for the Mariana's organisms is unclear. Polychlorinated biphenyls disrupt the hormone systems of some animals that dwell nearer the surface, and can also cause cancer, so the news is unlikely to be good. But what Dr Jamieson's work shows beyond peradventure is that no part of Earth's surface is insulated from the activities of Man.