If we are willing to spend tens of billion on research on Tocomak for fusion energy, why don't we do research on vertical tunneling to the Earth upper mantle layer for geothermal power ? The amount of energy is also limitless. It is only about 30~50 km deep. There are many countries in the ring of fire where the upper mantle layer should be even shallower. Today's technology already enable tunneling over hundreds of km, what is so difficult about just tunneling in a different direction? I imagine all we need to do is to ask the driver of the tunneling machine to steer downwards and make sure he keep going straight down, after a few months or years, he will reach the giant fire box, then he spray water on it, collect the steam to the steam turbine generator. The exhausted steam will rise up the tunnel, as it rises up, it cools and condense and flow back down to the fire box and the cycle can repeat endlessly without needing an extra drop of water and endless energy flow out ! Wouldn't that be nice and cheaper !