THEMES repurposes idle oil & gas wells to provide grid-scale geological long duration energy storage. We enable increased intermittent solar & wind generation on the grid to provide carbon-free firm dispatchable electricity cheaper than existing fossil generation. Our vision is to decarbonize electricity grids making it economically feasible to retire all base load and peaker fossil generation, replacing it with cheaper firm dispatchable 100% renewable electricity by repurposing oil & gas infrastructure. With over 2 million idle wells and 75% of the continental US having porous rock geology suitable for storage, Terawatt-hours of storage much cheaper than batteries is accessible.