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> It isn't practical to build mountains, so you need to find some natural mountains you don't mind cutting a big hole into

You don't even need a mountain. A hill will do:

https://en.m.wikipedia.org/wiki/Ludington_Pumped_Storage_Pow...

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When including inflation, it's delivered at a cost of 2 billion dollars for 19 GWh of storage. $200 per KWh isn't exactly good, it's not even on par with lithium ion batteries.

2016 generation: -752 GW/h

But since it’s not a generation facility and has a 70% efficiency, that implies it stored and returned 2506 GWh of electricity. At peak retail prices where I am in California (absurdly expensive) that’s worth over 1.5 billion dollars. Obviously you won’t have retail pricing, the market is different and you have transmission costs, etc. but the economics are still surprising. 200 $/kWh might be perfectly fine for a safe, low-maintenance, environment friendly storage that can be used with no time constraints.

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