drilling mud will definitely have to have some cooling properties but that's a hell of a strain on anything to suspend a pipe that long.
also i'm pretty sure it won't be like drilling into the roof of a cave. the rock will jsut get softer and softer until the rock with the lowest melting point become liquid I wouldn't imagine it would survive very far past that.
also you'd think that the time it would take to drill that deep the ground will have shifted and pinched the drill pipe.
2011 Chevy Colorado Z71 - 5.3L LH9 - Daily driver
2003 Mazda B4000 on 35s