The main driver of climate change is the release of fossil carbon, in the form of carbon dioxide, into the atmosphere. This anthropogenic carbon dioxide is mostly the result of the combustion of fossil fuels including natural gas. Natural gas is inexpensive, easily stored at national scale, and burns cleanly at high temperature, which is advantageous for thermodynamic efficiency. Hydrogen shares many of the desirable characteristics of natural gas but does not release carbon dioxide upon use. Global production of hydrogen is 100 million tons per year, most of which is used in the chemical, oil refining, and steel industries. More than 99 percent of hydrogen is produced by gasification of coal and refinery by-products or steam reforming of methane. Both these processes release large amounts of fossil carbon dioxide, so at present hydrogen makes no net contribution to the decarbonization agenda. However, in recent years surprising quantities of molecular hydrogen have been discovered in the subsurface. There are a number of mechanisms that could produce this geologic hydrogen by natural means. One of these reduces water to molecular hydrogen by oxidizing ferrous oxide (FeO) to magnetite (Fe3O4). Exploration geophysics applied to petroleum system analogues may be successful in finding commercially viable, naturally occurring hydrogen resources. However, in situ synthesis of H2, using the earth as a giant reactor, may furnish a more predictable route to large scale production. Bench scale experiments reportedly have successfully generated hydrogen from widely abundant earth materials such as olivine or basalt exposed to aqueous solutions at plausible subsurface conditions. Particularly interesting experiments simultaneously transformed basalt and carbonic acid to limestone, thereby potentially accelerating an important mechanism of carbon mineralization, while generating carbon-free hydrogen. In order to take these processes from laboratory to industrial scale many questions need to be answered: (1) What mineral or rock substrates are most advantageous? (2) What combinations of temperature, pressure, pH, and reaction time will prove to be practical? (3) Can modern methods of petroleum production enhancement be useful in hydrogen production and carbon mineralization in the subsurface? (4) Are these methods cost effective at scale?