Ongoing research into the prevalence of life-forms (predominantly bacteria and archaea) living deep in the Earth's crust is revealing them in previously unimagined quantities. Some researchers suggest that the biological diversity of deep-carbon organisms exceeds that of those on the surface.

The US-based Deep Carbon Observatory currently estimates that there are 15 to 23 billion tonnes of carbon tied up in deep life.

Many of the organisms can be classed as 'Extremophiles' - living at depths of up to at least 5km, with absolutely no light, and surviving temperatures of up to 137°C (see update below)

Measurements suggest that the continental subsurface hosts 2 to 6 × 10²⁹ cells.

Many questions remain about the sub-surface life - including :

Movement: How does deep life spread—laterally through cracks in rocks? Up, down? How can deep life be so similar in South Africa and Seattle, Washington? Did they have similar origins and were separated by plate tectonics, for example? Or do the communities themselves move? What roles do big geological events (such as plate tectonics, earthquakes; creation of large igneous provinces; meteoritic bombardments) play in deep life movements?

Origins: Did life start deep in Earth (either within the crust, near hydrothermal vents, or in subduction zones) then migrate up, toward the sun? Or did life start in a warm little surface pond and migrate down? How do subsurface microbial zombies reproduce, or live without dividing for millions to tens of millions of years?

Energy: Is methane, hydrogen, or natural radiation (from uranium and other elements) the most important energy source for deep life? Which sources of deep energy are most important in different settings? How do the absence of nutrients, and extreme temperatures and pressure, impact microbial distribution and diversity in the subsurface?"

Source : Deep Carbon Observatory [archived]

Update: Drilling operations of the Chinese Continental Scientific Drilling (CCSD) project in eastern China, have found 'continuous' microbial distribution up to a depth of 4,850 m, where the temperature was estimated to be around 137°C.

The research team note that "the depth limit of the terrestrial biosphere has yet to be determined". See: Geobiology, (early view) Feb 2021 (paywalled)

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