We think of life on Earth as existing on the skin of our rocky planet. We know there’s life almost everywhere we can think to look on the land and in the water. We don’t think of anywhere below the Earth’s surface as being a particularly hospitable place once you’re any distance underground. In fact scientists think that around 70% of the bacteria and archaea on Earth may be part of this ‘deep biosphere’, deeper than the deepest human mines. They’re even referring to it as the ‘dark biosphere’, or ‘bacterial dark matter’ — a huge proportion of life that exists is unknown to us, somewhere we can’t reach except with our deepest sampling equipment, and don’t yet understand, in a parallel with the missing mass of the wider universe.
We’ve barely begun scratching the surface of the Earth, in truth, and scientists are hopeful about what they’re going to find in the deep biosphere. There are organisms down there with such slow metabolisms that we wouldn’t normally think of them as having enough energy for life. In biology class, you’re often taught that literally all life on Earth is ultimately driven by energy from the Sun — that ultimately, all energy on Earth is derived from photosynthesis. It isn’t true: these organisms are far removed from sunlight, using energy harvested from rocks for power.
There’s also a hope that we can truly understand how life arose on Earth through studying these organisms. Maybe the deep biosphere, protected from the radiation and (comparatively) mercurial changes of the surface lands and oceans, is where we came from. At the very least, it may hold important evidence about just how early life arose and survived on an Earth very different to the one we know now.