The Kola borehole, drilled by the Soviets, is the deepest we've ever drilled, at 12.2 km.

I've alway wondered why they never went deeper. Turns out the answer is in hindsight obvious, but very unsettling:

> 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.

We could use the unexpectedly high temperatures as a heat source for geothermal energy. Make money selling the energy for ten, twenty years,... until the temperatures fall and the rock becomes rigid. Then drill another kilometer, funded from the geophysical exploration budget.

Temperature too high again? Have a long pause to make a little money from geothermal energy.

Geothermal plants can trigger earthquakes [1], so it's probably unwise to build a lot more geothermal plants to meet global energy needs without addressing that risk first, especially near populated areas.

[1] https://news.stanford.edu/2019/05/23/lessons-south-korea-sol...

To save you a search https://en.m.wikipedia.org/wiki/Kola_Superdeep_Borehole

My favorite thing about that hole is that we always imagine it to be human-sized or something, but it was as wide as a football or so.