Earth is teeming with life, but a new project shows most of it isn't where you'd expect. A decade-long study has now taken a census of one of the largest and least-understood ecosystems on the planet – the "deep biosphere" that extends several kilometers into the planet's crust. Among the finds are bizarre creatures that can survive at record depths, pressures and temperatures, and even "zombie" bacteria that may live (in a loose sense of the word) for millions of years at a time.

The project, known as the Deep Carbon Observatory, is the result of an international collaboration of scientists over almost 10 years. Data was collected from hundreds of sites across the world, with samples taken on land from mines and boreholes 5 km (3.1 mi) deep and up to 2.5 km (1.6 mi) under the seafloor.

Using that data, the researchers modeled these deep-Earth ecosystems, and estimated the amount of life down there. According to their calculations, up to 6 x10^29 cells (that's a 6 followed by 29 zeroes) live deep beneath the continental landmasses. When you include the life beneath the seafloors, there's approximately 15 to 23 billion tonnes of carbon biomass. The deep biosphere itself likely occupies up to 2.3 billion cubic km, which is almost twice the total volume of the planet's oceans.

"A decade ago, we had no idea that the rocks beneath our feet could be so vastly inhabited," says Isabelle Daniel, of the University of Lyon 1 in France. "Experimental investigations told us that microbes could potentially survive to great depth; at that time, we had no evidence, and this has become real 10 years later. This is simply fascinating and will surely foster enthusiasm to look for the biotic-abiotic fringe on Earth and elsewhere."

So, what kind of creatures live down there? The team says that all three domains of life – the broadest groups on the tree of life – are represented, with a genetic diversity at least as extensive as there is here on the surface. Two of those domains, bacteria and archaea, dominate the deep biosphere, which might contain as much as 70 percent of Earth's total amount of those groups. The vast majority of them are still completely unknown to science.

The creatures found in this deep biosphere consistently break records for the known extremes that life can survive under. The deepest-dwelling lifeforms have been discovered down to 5 km (3.1 mi) below land and 10.5 km (6.5 mi) below the ocean's surface.

But perhaps the strangest of all are bacteria that the researchers describe as "zombies." These organisms have life cycles on almost geologic timescales, millions or even tens of millions of years. But it's not much of a life – they don't really grow or undergo cellular division, instead focusing the little energy on hand into just barely maintaining their existence.

The scientists say we've barely begun to scratch the (sub)surface of the deep biosphere. Not only do most species remain unknown, but it's a mystery how they live, reproduce, move around, affect surface life and are affected by natural events like earthquakes and unnatural ones like fracking.

Researchers on the Deep Carbon Observatory project are presenting their findings at the American Geophysical Union meeting this week, which was described in recent papers published in Geobiology and Nature Geoscience, among others.

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