[srcmap]

Isn't the complexities related to making those caverns air and water tight for long period of time for long term energy storage requirements?

[count]

They don't need to be air/water tight for long term, just tight enough with a known leakage factor.

[edzillion]

But how cheap is it to determine the leakage factor? I suppose they can just pour in the equivalent water and see if it leaks?