Since the first discovery of chemolithotrophic “rock-eating” microbial life at the hydrothermal vents, our understanding that life is not simply a thin veneer on the earth's surface but may permeate deep into the subsurface of this planet has evolved rapidly. Geochemical processes of water-rock interaction are the key to providing energy and reducing power for subsurface chemoautotrophic microbial communities. The presentation will highlight work at underground sites in 2-3 billion year old Precambrian Shield rocks in South Africa, Canada and Finland where isotope geochemistry has identified large accumulations of free H ₂ gas, methane and higher hydrocarbons dissolved in saline fracture waters with residence times on the order of millions to billions of years. The integration of isotopic, geochemical and hydrogeological investigations with molecular and culture-based microbiology provides an exploration strategy to identify different zones of habitability in the Earth’s deep biosphere, and is informing exploration strategies for the search for life on Mars, Enceladus and Europa.