The deep earth really is a whole new (microscopic) world.

In Journey to The Centre of The Earth, Jules Verne describes a fantastical voyage where an expedition party encounter prehistoric creatures long since vanished from the surface.

As our knowledge of Earth Science grows, we find Verne's imaginative vision to have a grain of truth. However, the 'living fossils' are mostly bacteria and archaea, rather than a brontosaurus.

We think of life on this planet as existing only on the surface, and the oceans that mostly cover it. However, life extends underground into a space of over 2 Billion cubic kilometres, twice as large as the volume of all oceans on the planet, with 300 times as much carbon as all life on the surface.

70% of all life on this planet may be deep, deep beneath our feet, 5km down, or further, in conditions up to at least 120 centigrade. Far from the sun, they languish in ultimate sloth, following nothing faster than a geological drumbeat, living in suspended animation for countless millennia – perhaps millions of years – waiting for the right conditions to manifest, whereupon they spring back into activity.

So far, the prevalence of these depth-dwelling lifeforms seems to be quite uniform around the globe, suggesting that a lot of mixing has occurred and that these microbes may be very, very ancient indeed, perhaps forming when the Earth was still very warm, potentially pushing back our assumptions about when life originated substantially.

Did life begin elsewhere in the universe and rain down upon the Earth in meteorites? Or did up bubble upwards from the core, waiting for humanity to arise, to act as a fruiting body to carry its spores to the heavens?