Engineers also discovered, as they plowed past the first 14,800 feet (4,511 meters), that the rock had much more porosity and permeability. That, paired with the extremely high temperatures, made the rock behave more like a plastic than a solid, rendering drilling virtually impossible.
Once again the Russians are keeping their secrets…
At least they provided this tidbit:
https://science.howstuffworks.com/engineering/civil/kola-superdeep-borehole.htm#pt4
Plastic Earth confirmed.