Renewable electricity from solar and wind energy is converted into hydrogen by means of carbon-neutral electrolysis. This makes it possible to store a portion of the summer energy harvest as gas in our pore reservoirs for use in winter, or to combine the hydrogen with carbon dioxide (produced from biomass or industrial operations) and convert it into synthetic natural gas, in underground gas reservoirs, by means of a natural microbiological process. This creates a sustainable carbon cycle, and the naturally produced green gas is carbon neutral. By conversion we also mean the synthesis of CO2 and H2 to CH4 and the splitting of CH4 to hydrogen and carbon.