system
Given the following post, respond as if you're commenting on Reddit:
human
Why can't we just dig a deep enough borehole anywhere we need it to take advantage of geothermal energy?

Why is this not viable?  Dig a deep enough hole and you will get to useful heat levels.  I can certainly imagine all sorts of problems that would make this difficult, but nothing that would make it impossible.
gpt
Geothermal plants work by pumping(relatively) cold water 4-10km deep into the ground, heating it up, and retrieving hot steam at the surface. In the simplest designs, they use this steam to run a turbine. Unless the area is around active tectonic zones, drilling 4-10km deep won't be hot enough to make the steam you want. But than you ask, "it's free heat, who cares about efficiency?" Eventually, pumping increasing amounts of water 10 kilometers into the ground is taking up more energy than it is worth. 

Edit: And as the other poster said, geothermal heat pumps are very viable. Put some pipes 50 feet into the ground and use it as a thermal reservoir, instead of the ambient air. After  about six feet, the ground stays relatively constant year around. You can use the warm ground in the winter to heat up your house, and use the cool ground to cool your house in the summer.