Two topics have not been covered yet:<p>1) Because its more spectacular there's been discussion of exploding substations and arcing power lines. Which is true.<p>However a more practical HN topic would be "we all know" that inter building LAN networking should be optical for power isolation reasons, but "we all know" that often you can get away for years with copper network cables over a short enough distance if they're on a similar enough electrical feed. If you live in Florida, world capitol of thunderstorms, the wiring and switch ports won't survive a week, but maybe northern yankees can do copper inter building networking for years between lightning related failures. Anyway, yesterday, if you lost some ethernet switch ports or entire switches connected to 100 meter copper lines over long distances maybe to an outbuilding or up a tower or to the roof for cameras or something, well, you kinda know why now. Of course you probably lose 0.1% per year of hardware anyway under normal conditions so maybe losses were mere coincidence... or was it? There's probably a dollar value to blown ethernet ports related to that storm; can't say if its $10K or $10M worldwide but I'm sure at least SOME money was lost in hardware and lost productivity.<p>2) Aside from bulk power transport, and wired signalling, a third application of ground current is archaeological / geological resistance surveys.<p><a href="https://en.wikipedia.org/wiki/Electrical_resistance_survey" rel="nofollow">https://en.wikipedia.org/wiki/Electrical_resistance_survey</a><p>The idea is underground "stuff", perhaps geology layers, perhaps ancient ruins, has varying DC resistance compared to typical background dirt and rock, and we have the technology to measure and map those electrical anomalies and therefore determine things about underground stuff. Cool. When the background electrical and magnetic fields are quiet/constant -ish, anyway.<p>I've always wanted to participate in one of those, never got the chance. Anyway weird earth currents mean its probably pretty hopeless to try gathering research data during a geomagnetic storm. Yeah I know the 4-wire technique is supposed to help and differential measurements and all that, but the gear is designed for normal conditions not crazy storms so there seems no way you're gonna get good data during a severe storm.<p>I mean, even the most obvious non-ground measurement problem is even differential GPS is going to get annoyed at massive ionospheric disturbances so you're going to have time/location noise in the data, if nothing else. I wonder how bad the geomagnetic weather has to be to keep construction civil engineering surveyors home for the day, probably pretty bad indeed, but I bet a lot less and a lot more likely to happen than it takes to "short out substations" and vaporize high voltage lines or whatever doomsayer stuff.<p>Aside from DC resistance measurements, I know the same people do magnetic surveys and likewise having a huge geomagnetic storm as a noise source would seem likely to degrade the gathered data.