Why is it so hard to drill off Earth?

Humans have been digging underground for millennia—on the Earth. It's where we extract some of our most valuable resources that have moved society forward. For example, there wouldn't have been a Bronze Age without tin and copper—both of which are primarily found under the ground. But when digging under the ground on celestial bodies, we've had a much rougher time. That is going to have to change if we ever hope to utilize the potential resources that are available under the surface. A paper from Dariusz Knez and Mitra Kahlilidermani of the University of Krakow looks at why it's so hard to drill in space—and what we might do about it.