Heat
The first 3 kilometers of their drilling attempt had been fairly predictable. The temperature was about what the scientists expected. But then something changed that they couldn’t have foreseen beforehand. The temperature started to rise much faster than expected. The closer the researchers got to their target, the further the mercury rose. When they reached around 15 kilometers, they were already at a temperature of 180°C. That was 80°C warmer than their predictions!
But that wasn’t all. In addition to the heat, there was another discovery that messed up the whole investigation. What they found at the bottom of their insanely deep borehole was the reason they decided to abandon their attempt for good.