But where? In Finland, at least, the land is relatively flat when compared with Norway and Sweden, and with a large rural population there aren't really any good locations.

In my local area, we had major flooding this spring because the hydro plant operators were sleeping on the job (or whatever they did instead of regulating water levels). And that was a simple 2m increase in water levels.

NO/SE have some more geographically suitable locations, but last time I checked, flooding them was considered too environmentally destructive too the local environment.

> You could use the ocean for the bottom level and an artificial reservoir for the top level. You're not going to noticeably affect ocean levels.

Then you have to deal with the problem of sea water corroding everything it touches.

> You might pump 10 billion litres of water, which is .02% of the volume of Mjøsa.

It's not the amount of water that you pump, it's the amount * the elevation delta. Where are you planning on getting the elevation delta from?

Neither of these challenges is technically insurmountable, but this is a field where capex + opex/KWH is everything.

I imagine the thaw/freeze cycle would be hell on the equipment to run pumped hydro storage.

Are there extant succesful examples of pumped hydro in cold regions?

They tried in southern Finland not long ago. At great expense and spending a lot of time they managed to drill down 6-7 km until they figured out that the porosity of the rock down there was so poor that it was impossible to make the project economical, so it was cancelled. The idea was to pump this heat directly into the district heating grid.

Either fusion or drill baby drill is necessary. Watt’s steam engine was absolutely horrible, but it was the worst steam engine ever built. If Finland builds the worst deep geothermal ever that still works, we can hope for better ones.

Yeah I know drilling through ~8-10 kilometers of rock is kinda hard… they know, they tried, maybe it now is a good political climate to try again?