WASHINGTON, Dec. 10 (Xinhua) -- Bacteria and unknown forms of life in Deep Earth, or so-called subterranean microbial "dark matter", are hundreds of times greater than the carbon mass of all humans on the surface.
A 10-year study presented on Monday at the American Geophysical Union's annual meeting reported how much and what kinds of life exist in the deep subsurface under the greatest extremes of pressure, temperature, and low nutrient availability.
The size of the deep biosphere amounts to 2 to 2.3 billion cubic km, or almost twice the volume of all oceans, and the carbon mass of deep life totals 15 to 23 billion tons, according to the study.
Scientists with the Deep Carbon Observatory drilled 2.5 km into the seafloor and sampled microbes from continental mines and boreholes more than 5 km deep.
They found 70 percent of Earth's bacteria and archaea live in the subsurface and among them are millions of distinct types, most yet to be discovered.
The genetic diversity of life below the surface is comparable to or exceeds that above the surface, according to the study.
A candidate for Earth's hottest organism is a single-celled organism living in hydrothermal vents on the seafloor, growing at 121 degrees Celsius, the researchers said.
The record depth at which life has been found in the continental subsurface is approximately five km and the record in marine waters is 10.5 km from the ocean surface.
"Exploring the deep subsurface is akin to exploring the Amazon rainforest. There is life everywhere, and everywhere there's an awe-inspiring abundance of unexpected and unusual organisms," said Mitch Sogin with the Marine Biological Laboratory and co-chair of the Deep Carbon Observatory's Deep Life community comprising more than 300 researchers in 34 countries.
"Expanding our knowledge of deep life will inspire new insights into planetary habitability, leading us to understand why life emerged on our planet and whether life persists in the Martian subsurface and other celestial bodies," said Fumio Inagaki, Japan Agency for Marine-Earth Science and Technology.