A drill chews downward, making about a tall adult's height of progress per day on average. The Kola drilling project lasts 22 years. Then hot rock starts closing the hole behind it.
Work began in 1970 on the Kola Peninsula. Scientists wanted to touch rocks they had only mapped with earthquake waves. Mud pumped down a pipe drove the drill at the bottom and helped bring samples up.
The rocks surprised them. A boundary they thought marked a change from granite to basalt held no expected basalt. Cracks and changed rock had fooled the map. Water turned up where they had expected dry stone.
Heat finally made the job too hard. Rock crept inward when the drill came out. In 1989, the hole reached 7.6 miles (12,262 meters): deeper than Everest is tall, with about 3 kilometers to spare.
You could bring up a cylinder of ancient gneiss, a banded rock changed by heat and pressure, and hold a real piece of that depth. Below it, we still do not know every rock the waves pass through. The Clock dates the old basement rock, not the drilling. Our deepest samples stop far short of Earth's center.
