You could theoretically store energy by winding springs and making use of it by letting them unwind (by spinning a generator or whatever). Toy-car style. This

You could theoretically store energy by winding springs and making use of it by letting them unwind (by spinning a generator or whatever). Toy-car style. This doesn't involve digging a deep hole in the ground and hope that the earth keeps it level. A drawback would be the hazards that heavy tension/forces bring with them.