Radioactive material from last century's nuclear weapons testing has reached the very bottom of the ocean.
Scientists found contaminated 'bomb carbon' inside creatures living at the bottom of the Mariana Trench - at a few metres shy of 11km, the deepest spot in the world.
"Although the oceanic circulation takes hundreds of years to bring water containing bomb [carbon] to the deepest trench, the food chain achieves this much faster," said lead researcher Ning Wang, geochemist at the Chinese Academy of Sciences in Guangzhou.
"Human activities can affect the biosystems even down to 11,000 metres, so we need to be careful about our future behaviour."
The amount of radioactive carbon-14 in the atmosphere doubled during the Cold War arms race of the 1950s and '60s, before reducing to only 20 percent above pre-Cold War levels by the 1990s.
But rather than vanish, it merely went into the ocean. Creatures at the surface which absorbed it, after they die, sink - where they're eaten by bottom-dwelling species.
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The bomb carbon was found in the muscle tissue of deep-sea crustaceans.
"It's not expected, but it's understandable, because it's controlled by the food chain," said Wang.
The only country which has carried out nuclear testing in the past 20 years is North Korea.
The study was published in journal Geophysical Research Letters.
Last year a plastic bag made headlines when it was found at the bottom of the Mariana Trench.
Newshub.
