I recently read something about how all our energy crises could be addressed cheaply, safely and sustainably by gravity drives installed in old mines. What I didn't read was any mention of anyone actually getting started on this.
Also, it's clear that the author never saw Event Horizon.
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@darryl Currently the preferred method seems to be Pumped Hydro, but using old mines seems like a good option where water is scarce. https://en.wikipedia.org/wiki/Pumped-storage_hydroelectricity
@ambulocetus This is fascinating stuff.