Not hot enough to melt, but heat definitely reduces structural integrity of materials. And pressures on the rock so deep are immense. This article linked from W

Not hot enough to melt, but heat definitely reduces structural integrity of materials. And pressures on the rock so deep are immense. This article linked from Wikipedia seems to confirm that the hole tended to close itself when they were drilling: "At that level of heat and pressure, the rocks began to act more like a plastic than a solid, and the hole had a tendency to flow closed whenever the drill bit was pulled out for replacement." https://www.damninteresting.com/the-deepest-hole/ https://www.damninteresting.com/the-deepest-hole/