The planet beneath our feet
From 2009 to 2019, a global research collaboration called the Deep Carbon Observatory (DCO) worked to transform our understanding of carbon's role inside the Earth. This ten-year program involved over 1,200 scientists from 55 countries, organized into four communities: Deep Life, Deep Energy, Reservoirs and Fluxes, and Extreme Physics and Chemistry. They explored life's limits by drilling 2.5 kilometers into the seafloor and sampling from continental mines more than 5 kilometers deep.
The DCO's most startling finding was the sheer scale of the deep biosphere. This subterranean area, which extends kilometers into the planet's crust, has a volume of 2 to 2.3 billion cubic kilometers—nearly double the volume of all the world's oceans. Within this space exists a vast community of life, primarily bacteria and archaea. The total carbon mass of this subsurface life is estimated to be between 15 and 23 billion tonnes (petagrams). This is 245 to 385 times more than the carbon mass of all humans on the surface. About 70% of Earth's total bacteria and archaea live underground, hidden from the sunlit world.
Life in the dark
Life in the deep biosphere exists under conditions of extreme pressure and temperature, with no light and few nutrients. Some microbes have been found living at temperatures up to 121°C (250°F). Life has been detected as deep as 5 kilometers below the continental surface and 10.5 kilometers below the ocean surface. Organisms in this environment often have metabolisms that are millions of times slower than those on the surface. Some of these so-called "zombie" bacteria live on near-geologic timescales, using the little energy they have just to maintain their existence rather than to grow or divide.
One of the most remarkable examples is Candidatus Desulforudis audaxviator. This bacterium was discovered living 2.8 kilometers down in a South African gold mine, in a completely self-contained ecosystem. It draws its energy not from the sun, but from the radioactive decay of uranium in the surrounding rocks. This process splits water molecules, creating chemical food sources that the bacterium can use. D. audaxviator has all the genetic tools it needs to survive alone, including the ability to extract carbon from its environment and fix its own nitrogen.
The DCO also quantified the Earth's total carbon budget. An estimated 1.85 billion gigatonnes of carbon are stored inside the planet. Only 43,500 gigatonnes, or about 0.2 percent, reside on the surface in the atmosphere, land, and oceans. The DCO's research showed that carbon is continuously exchanged between the deep Earth and the surface through processes like volcanic eruptions and the subduction of tectonic plates. Human activities release 40 to 100 times more carbon dioxide annually than all of Earth's volcanoes combined.