I was thinking about how you might grow plants inside without sunlight powered only by LEDs. But to get the energy for the LEDs, one might drill a hole down into the Earth's magma and put a steel pole. Then the heat from the magma would heat up the pole and you might be able to generate electricity at the top from the heat.
I was wondering, if the pole was for example 1cm in diameter, how deep would you have to drill it to generate electricity for one LED?
This assumes that the heat would make it's way to the top of the steel pole without dissipating. Hopefully it would as steel conducts heat better than rock.