Pumped hydro is gravity storage. It works. Running train cars loaded with rock aggregate up and down disused mountain rail tracks could be a good idea, depending on how you get the power out (third rail?), but seems like it would be expensive to keep working. Running a multi-thousand-ton weight up and down a disused mineshaft seems a certain winner, anywhere you happen to have such a shaft at hand. Pumping air into a bag anchored to the ocean floor. Winching an air tank down to a pulley anchored to the ocean floor.

Winners in energy storage will be distinguished from the field of failures along axes of cost, secondary usefulness, compatibility with existing infrastructure, service lifetime, energy capacity, power capacity, environmental impact, simplicity, and efficiency. For example, synthetic chemical and liquified air media excel at secondary usefulness, compatibility with existing gas turbines, and cheap tankage (unlimited capacity), at the expense of cost and efficiency. Their main cost is for synthesis capacity, measured in watts. Low efficiency is tolerable in exchange for other merits.

Energy Vault is a certain failure on the basis of cost alone, even without considering its other low scores. Constructing condominiums for concrete blocks is just extremely expensive, even before counting predictably very high maintenance cost, for minuscule storage capacity.

Simplicity is the most frequently neglected, but often most valuable quality. The best storage systems will have just one moving part used for both storing and extracting energy. Next best, two.