Carbon plays an unparalleled role in human life. It is the element of life, providing the chemical backbone for all essential biomolecules. Carbon-based fuels supply most of society's energy, while small carbon-bearing molecules in the atmosphere play a major role in Earth's variable and uncertain climate. Yet in spite of carbon's importance, scientists remain largely ignorant of the physical, chemical, and biological behavior of carbon-bearing systems more than a few hundred meters beneath Earth's surface. Little is known about how much carbon is stored in the Earth as a whole, how deep reservoirs form and evolve, or how carbon moves from one deep repository to another. Further, scientists are largely ignorant of the nature and extent of deep microbial ecosystems, which by some estimates rival the total surface biomass.