Store renewable energy in underwater balloons
The greatest challenge of renewable energy is finding a sustainable way to store power during off-peak production. Underground air balloons can offer a cheap, effective, and green energy storage solution.
You could also store energy in the water!
https://www.dw.com/en/swiss-water-battery-boosts-europes-energy-storage-plans/a-63923662
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