It might interest people that the soon-to-be previous biggest thermal energy store is also located in Finland, under the island of Mustikkamaa in the capital city of Helsinki. The city heating company Helsingin Energia "charges" the store by pumping heat out of sewage in summer. I think it was about 10 gigawatt-hours and it's not pressurized, so water can only reach 90 C over there.
(A side note: if you allow water at 140 C to boil in a controlled manner, you get steam, which can also produce electrical power, although probably in a suboptimal manner.)
Finnish bedrock seems more suitable than average rock for such ventures (which I would call "artificial geothermal energy") - granite is a poor thermal conductor and a reliable rock for making caverns.
I hope it goes well. :)