>Well costs are ~quadratic in depth.

Anyone know as to why this is the case? At first pass it seems like your cutting tools should wear out in proportion to depth. But I suppose as things get warmer tool wear becomes faster. What types of tool materials are used for rock boring? Diamond? Tungsten carbide? Is is more of a "conventional machining" process, or an abrasive / water jet? Or explosive like in hard rock mining? What else has an effect on cost for deep holes? Shaft length gets longer, so the twist becomes more of a problem if your are driving rotating bits from the top? Does it have to do with removing the material from the bottom of the hole to the top? Others?