In recent years, multiple research groups have tremendously advanced understanding of subseafloor sedimentary life. Microbes in subseafloor sediment are now known to be abundant, diverse and characterized by extraordinarily low mean rates of activity. Some discoveries challenge our sense of what is possible. For example, per-cell energy fluxes are far below the rates believed necessary for reproduction. What mechanisms might allow cells to reproduce at such low rates? Or do many of them live for millions of years without reproducing? Bulk population studies show that a very large fraction of these cells is active. However, we know essentially nothing about cell-to-cell variation in respiration, biomass turnover or reproduction. Furthermore, we do not clearly understand how organic-fueled respiration can persist for tens of myrs at very slow rates. Subseafloor community structure is largely unexplored. We have very limited understanding of the ways in which subseafloor microbes compete and almost no understanding of how they cooperate. Roles of viruses, eukaryotes, resting stages and bacterial spores in subseafloor ecosystems are largely unknown. The proximate causes and ultimate consequences of natural selection in subseafloor communities remain unknown. For the most part, we do not yet know the genetic potential of subseafloor microbes, the extent to which their potential is expressed, or the conditions under which they are expressed. The actual limits to subseafloor life are not yet known. Advancing understanding of these issues will yield fundamental insight into the nature of life.