And the source of geothermal energy is technically nuclear decay (fission) from fissile materials in the crust+mantle

So if we had magma 100-1000 meters under our feet that was easily accessible and shot out of the ground with a shovel, I agree; we'd probably already be using the mantle for energy since the 1900s instead of oil - lol

Though since you're playing that mod pack, the real energy gains are made by drilling a portal into hell (nether) and that's the point where you've gotta realize Minecraft solutions might not apply in the real world :>

How To Fix Traffic Forever
https://www.youtube.com/watch?v=Ie5hOGMsgNQ

been fiddling around with this a while but it's finally finished!

The Catastrophic Failure of Europe's Newest Power Plant. What Happened?
https://www.youtube.com/watch?v=gAr6uJ5NS6o