Forgive me for a lack of complete understanding about mining and drilling. I have worked at a couple mines, but only for brief periods, either back when I was a student, or doing work not exactly related to mining.

You say that drilling below the overburden would be more effective. I was under the impression that the overburden is often the softest stuff to get through? Isn't overburden often anywhere from ten to a hundred feet of soil, gravel, and looser rocks, before encountering more contiguous rock strata underneath that might contain useful ores?

I guess I really should learn more about mining. The only places that I've worked were two different limestone quarries (for cement plants) and a copper mine with a fair amount of gold as a byproduct.