Earth is not the home you think it is. Far below the scant surface spaces we inhabit, the planet is teeming with an incredibly vast and deep 'dark biosphere' of subterranean lifeforms that scientists are only just beginning to comprehend. Hidden throughout this subsurface realm, some of the world's deepest and oldest organisms thrive in places where life shouldn't even exist, and in new research, scientists have quantified this 'dark matter' of the microbial world like never before.