I've even heard of a technology for pushing compressed air in abandoned mines with surplus electricity, to be let out through turbines when there's need for pow

I've even heard of a technology for pushing compressed air in abandoned mines with surplus electricity, to be let out through turbines when there's need for power. We do all of the above (except the compressed air thing and superconductors) here in Finland and between neighboring power grids. Of course there is room for improvement.