But the higher pool - or pools - of water needn't be at ground level, and they could be cisterns, shaded or semi-shaded and so on, maybe set into the sides of the upper crater, or whatever.

Equally, it needn't be a "mega" project to start with.

As for wind resource and solar resource in the area, I have no idea either.

It's just the idea of a f..k off big hole as a battery that I like!

Me, I'd just chuck the idea at a few engineering schools as a competition.

The "best" solution gets the ET prize...

"The future is already here -- it's just not very evenly distributed" William Gibson

But I'm only looking here at the Kimberley "Big Hole" (not any other mine) which ceased operations about 100 years ago, and has since part-filled with water, under which is a combination of collapsed mine, and material chucked into the hole.

It looks pretty stable now, and as far as I know, it's full of water year around, which as it's 180 metres down, and relatively shady, would not be surprising.

Frankly, in engineering terms, the concept is pretty trivial. It's not like you're hollowing out a mountain, as at Dinorwic.

That's the point.

But if there's no need, current or future for energy storage in the vicinity, then the idea would never even be a starter.

"The future is already here -- it's just not very evenly distributed" William Gibson