The deep subsurface harbours the unseen majority of microbial diversity on Earth, and a significant proportion of the planet’s biomass. Understanding how life operates in these extreme environments highlights the roles these microbes play in biogeochemical cycles of global significance, such as the carbon cycle. Such knowledge also helps to define the limits to life on Earth. However, until recently little was known about the diversity and function of microbial life in the deep biosphere. With the recent advent of affordable sequencing technologies and ever sophisticated tools to unpick the genetic code, we now know that the deep subsurface harbours an active and highly diverse biosphere. In this talk I will present data from subsurface environments that highlight the diversity of deep life, and its contributions to biogeochemical cycling. Examples will include evidence for an active carbon- and sulfur-cycling microbial community in a deep borehole in Greenland, a novel genus of bacteria recovered from a 2.5 km deep shale gas well, and a glimpse of the microbial life deep beneath our feet in the North of England. With these examples I will demonstrate the power of cutting-edge genomic tools for uncovering the ‘rules of life’ in the subsurface, and highlight the significant potential to put subsurface microorganisms to work for the good of the environment and society.