What's really old, is the knowledge that the Earth's core is hot. The crust has
a known thermal gradient, and is moving that heat at conduction-through-dirt rates
consistent with that gradient. You'd have to run a heat engine to bypass a LOT
of vertical distance, in the downward direction, to tap that.
And, as you tap it, the bottom of your heat engine gets cold. That means
it's not sustainable, unless you
find a magma flow and tap the heat of a river of molten rock.
The sustainable 'amount of energy' (really, power) you can get from the planet in
this way is about 1/16 watt per square meter. Mile-deep holes are more
expensive than solar cells, and sunlight is delivering 'way more energy than that anyhow.