Why did the deepest hole ever drilled stop at 12 km?
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Answer: Rock got so hot it flowed back into the hole
Rock got so hot it flowed back into the hole ✓ — Correct! At 12.2 km down, rock at 180°C started behaving like warm plastic — it flowed back into the borehole between drilling sessions. The Kola Superdeep Borehole (1970-1992) wasn't beaten by hardness; it was beaten by rock that refused to stay carved.
The drill hit an impassable layer of diamond — Not quite. Diamonds form at 150+ km depth — far beyond any drill. The Kola hole passed through ordinary Archean gneisses. The obstacle wasn't a mineral; it was temperature turning hard rock soft.
They reached the mantle and couldn't drill through — Not quite. Under continents, Earth's crust is 30-50 km thick. The deepest borehole barely made it a third of the way. The mantle was never in range — heat beat the drill long before geology could.
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