Generating methane from surplus renewable electricity via Power to Gas Technology opens the perspective of large scale seasonal storage, transportation, distribution and utilization of renewable energy with existing infrastructures without technical limitations – as still existing for hydrogen. The project investigates the in-situ microbial methanation of carbon mono- and dioxide using hydrogen in depleted natural gas reservoirs, and aims to develop a process chain for its industrial utilization. Laboratory experiments in the course of the Underground Sun Storage research campaign strongly indicate that microbial consortia present in depleted biogenic gas reservoirs are capable of using hydrogen for the formation of methane. Such a technology would make it possible to generate and convert large amounts of renewable energy, both in Austria and in areas with a high potential (wind from Patagonia, sun in desert areas) and export this energy to densely populated areas with a lower renewable energy generation potential. CO2 utilization in that process leads into a circular flow carbon economy.