Geothermal is good, but the extractable energy is calculated by the temperature differential:

Very simple:
Tin - Tout = ∆°C

Then each element has a known value of energy it will carry, for instance:

Liquid water: 4.184 J/∆°C

So say you have a geothermal vent which is 85°C, and you pipe room temperature water down to it (20°C):

85-20= ∆65°C × 4.184 = 272 joules per gram

So in one liter of water piped down, you'd get 272 kJ (joules are an SI unit of pure energy).

Let's say you want to run an iPhone 14S Max on that energy. It has a 10 Wh battery.

To convert Wh to joules, multiply by 3600 (seconds in an hour).

So the smartphone needs 36 kJ of energy to run. One liter of the naturally boiled water, if extracted at 100% efficiency, would power your smartphone for (272÷36 = 7 and ½ days).

Of course, this assumes the natural hot spring is 85°C. There's not many places in the world that's true. Let's say the groundwater is 22°C, instead of 85°C, just slightly hotter than room temperature.

With the same math, you can now only extract 8.3 kJ per liter of pumped water, not even enough to partially charge an iPhone.

So the "geothermal spots" assuming you had infinite money, and are wise, are limited to the "ring of fire" on the Earth.

You would need to drill extremely deep anywhere else to see an appreciable temperature difference

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 :>