The Kola borehole, drilled by the Soviets, is the deepest we've ever drilled, at 12.2 km.
> The unexpected decrease in density, the greater porosity, and the unexpectedly high temperatures caused the rock to behave somewhat like a plastic, making drilling nearly impossible.
Temperature too high again? Have a long pause to make a little money from geothermal energy.
[1] https://news.stanford.edu/2019/05/23/lessons-south-korea-sol...