Microbes are thriving in surprising numbers at the deepest spot in the oceans, the 11,000-metre (36,000 ft) Mariana Trench in the Pacific, despite crushing pressures in sunless waters, scientists said.
Dead plants and fish were falling as food for microscopic bugs even to the little-known hadal depths, parts of the seabed deeper than 6,000 metres and named after Hades, the god of the underworld in Greek mythology, they said.
The presence of life in the trench also shows how the greenhouse gas carbon dioxide, vital for the growth of tiny marine plants at the ocean surface, can eventually get buried in the depths in a natural process that slows climate change.
A Danish-led team of scientists, using a robot to take samples, found double the amount of bacteria and other microbes munching away on debris at the bottom of the Mariana Trench in the western Pacific than at a nearby site 6,000 metres deep.
"It's surprising there was so much bacterial activity," said Ronnie Glud, of the University of Southern Denmark and lead author of the study in Monday's edition of the journal Nature Geoscience.
"Normally life gets scarcer the deeper you go. But when you go very deep, more things start happening again," he told Reuters of the report that also involved research institutes in Scotland, Greenland, Germany and Japan.
The finding backed up a theory that dead plants and fish falling onto the steep sides of the Mariana Trench often slide to the bottom to form a "hot spot" for microbes. Earthquakes also trigger mudslides that carry debris down.
The Mariana Trench is five times longer than the Grand Canyon and could easily swallow the world's highest mountain Mount Everest, which stands 8,848 metres tall.
Life has been detected at the bottom before, but its extent is little known. The scientists' video cameras also spotted a few shrimp-like crustaceans at the bottom of the trench.
"It's most likely that more carbon is deposited" in the hadal depths than previously believed, Glud said.
"We have a small exotic piece of the puzzle which has never been studied before," Glud said of the way that the oceans recycle or bury carbon.
Only about 2 percent of the world's oceans are deeper than 6,000 metres.
Until now, scientists had suspected that life in most of the ocean depths, where waters are just above freezing, was severely limited by a lack of food.
Only about one or two percent of living material in the upper waters is expected to sink even to the average ocean floor depth of 3,700 metres, the study said. Most food gets scavenged and carried up towards the surface before it falls so deep.
And water pressure at the bottom of the trench is about 16,000 lbs per square inch (1,125 kg per sq cm), about the same as being stepped on by an elephant wearing high-heeled shoes.
The scientists were also studying the genetic makeup of the microbes, living in temperatures just above freezing.
The ability to survive crushing depths may mean they have enzymes that could be used by industries that use high pressures, ranging from fermentation to oil and gas.
The bottom of the Mariana Trench was first reached by scientists in a submarine in 1960. Film director James Cameron also descended in 2012 and reported few signs of life.