In the last three decades we have learned a great deal about microbes in subsurface environments. Once, these habitats were rarely examined, perhaps because so much of the life that we are concerned with exists at the surface and seems to pace its metabolic and evolutionary rhythms with the overt planetary, solar, and lunar cycles that dictate our own lives. And it certainly remains easier to identify with living beings that are in our midst, most obviously struggling with us or against us for survival over time scales that are easiest to track using diurnal, monthly or annual periods. Yet, research efforts are drawn again and again to the subsurface to consider life there. No doubt this has been due to our parochial interests in the resources that exist there (the water, minerals, and energy) that our society continues to require and that in some cases are created or modified by microbes. However, we also continue to be intrigued by the scientific curiosities that might only be solved by going underground and examining life where it does and does not exist.
But really, is life underground just a peculiarity of most life on the planet and only a recently discovered figment of life? Or is it actually a more prominent and fundamental, if unseen, theme for life on our planet? Our primary purpose in this chapter is to provide an incremental assembly of knowledge of subsurface life with the aim of moving us towards a more complete conceptual model of deep life on the planet. We aim to merge the consideration of the subsea-floor and the continental subsurface because it is only through such a unified treatment that we can reach a comprehensive view of this underground life. We also provide some thoughts on a way forward with what we consider to be interesting new research areas, along with the methods by which they might be addressed as we seek new knowledge about life in this Stygian realm.