At a Glance

Researchers found toxic methylmercury in animals from the Marianas Trench.Earlier research indicated mercury pollution stayed closer to the ocean's surface.It sinks to the ocean bottom in the carcasses of fish and marine mammals.

Toxic, human-caused mercury pollution has been found in the Marianas Trench, the deepest part of the ocean.

The findings from two independent studies could have far-ranging implications for how mercury can be concentrated in the food chain, experts said.

The studies have been presented at the . The presence of manmade and natural methylmercury, a toxic form of mercury easily accumulated by animals, in fish and crustaceans in the 36,000-foot-deep Marianas Trench indicates the pollution is much more widespread than previously thought.

(MORE: Mercury Levels Rising in Fish Because of Climate Change and Overfishing, Study Finds)

“This is a surprise. Previous research had concluded that methylmercury was mostly produced in the top few hundred meters of the ocean. This would have limited mercury bioaccumulation by ensuring that fish which forage deeper than this would have had limited opportunity to ingest the methylmercury. With this work, ,” Ruoyu Sun, who led researchers from China's Tianjin University, said in a statement.

Ruoyu Sun's team used the submersible "Deep Sea Warrior" to explore the Marianas Trench in the Western Pacific Ocean.

(Ruoyu Sun and IDSSE-CAS)

Sun's team used deep-sea vehicles to collect creatures and sediment from the Marianas Trench in the Western Pacific Ocean.

"We are able to present unequivocal mercury isotope evidence that the mercury in the trench fauna originates exclusively from methylmercury from the upper ocean," Sun said. "We can tell this because of the distinctive isotopic fingerprint which stamps it as coming from the upper ocean."

A separate group led by Dr. Joel Blum of the University of Michigan sampled fish and crustaceans from the Marianas Trench and the Kermadec Trench near New Zealand.

Blum's team determined the methylmercury in the animals fell on the ocean surface from rain in the atmosphere. It sinks to the ocean bottom in the carcasses of fish and marine mammals as well as in small particles, Blum said.

"This work shows that human-released mercury has reached and entered food webs in even the most remote marine ecosystems on Earth," Blum said in a statement. "This better understanding of the origin of mercury in the deepest reaches of the ocean will aid in modeling the fate of mercury in the atmosphere and oceans."

Mercury can be emitted when coal or oil are burned and during industrial processes like mining and manufacturing. It can also be produced by natural events like volcanic eruptions and forest fires. It poses health risks to wildlife and humans when it becomes methylmercury.

(WATCH: Species Discovered in Marianas Trench Named for Plastic in Its Gut)

When mercury gets into the ocean, tiny microbes change it into methylmercury. From there it gets into the food web — from zooplankton to crustaceans and small fish and then into larger fish. The mercury becomes more concentrated as it rises in the food chain. Humans ingest it when they eat seafood.

In humans, mercury can cause neurological damage, cardiovascular risks and other health effects. It is particularly dangerous to developing fetuses.

Ken Rubin, an Earth sciences professor at the University of Hawaii who was not involved in the research, said, "We are now learning from these two studies that the effects of this deposition (of mercury) have spread throughout the ocean into the deep sea and the animals that live there, which is yet another indicator of the profound impact of modern human activities on the planet."