The annual 70 million tons' CO2 injection separated from flue gas of fossil fueled power plants into aquifer may reduce Japan's artificial carbon dioxide emission as much as 6% with annual cost of 420 billion yens that match 1, 200 yens/c carbon tax. Geological sequestration of CO2 is a technically and economically feasible option for mitigation of greenhouse gas emission.Carbon dioxide injection under gas-hydrate-filled layers or under permafrost layers can realize the greenhouse gas mitigation and recovery of unused natural gas. Autogenous sealing of CO2 in deep and cool aquifers assures virtually complete and practically unlimited subsurface containment of CO2.Chemoautotrophs fix carbon dioxide in deep aquifers even without sun light. Methanogens can convert CO2 into methane in deep anoxic aquifers. Biogenetic restoration of subsurface hydrocarbon deposits is expected in CO2injected aquifers after tens or hundreds of years. Microbiological recycling of CO2 in aquifers is an attractive future technology for energy-short countries such as Japan. Important key for underground CO2 recycling is hydrogen supplies owing to microbial decomposition of organic matter and from geochemical water-rock interaction in deep rock formations.