I couldn't see in the article - is this using natural caverns or did they excavate? I know the pilot versions of this tech generally use old salt-caverns, like

I couldn't see in the article - is this using natural caverns or did they excavate? I know the pilot versions of this tech generally use old salt-caverns, like the one in Germany. I'm actually pretty excited about this tech - it seems like solar and wind are getting cheap faster than batteries will be able to meet the needs for grid-scale energy storage, so a cheap-but-inefficient energy storage tech is an exciting prospect. Massively overbuild the solar/wind and use these things to defer the overflow.