2 links have been added on this idea.
  1. You could also store energy in the water!
    https://www.dw.com/en/swiss-water-battery-boosts-europes-energy-storage-plans/a-63923662

  2. Underground energy storage balloons are the solution to ending environmental pollution by lithium batteries.
    https://www.sauder.ubc.ca/news/using-underwater-balloons-store-energy