Geothermal hot dry rock drilling at Fenton Hill in northern New Mexico encountered problems of catastrophic lost circulation in cavernous areas of limestones in the Sandia Formation, severe corrosion due to temperatures of up to 320/sup 0/C, and torque problems caused by 35/sup 0/ hole angle and the abrasiveness of Precambrian crystalline rock. The use of polymeric flocculated bentonite fluid, clear water, fibrous material, dry drilling, oxygen scavengers, a biodegradable lubricant mixture of modified triglicerides and alcohol, and maintenance of a high pH, were some of the approaches taken toward solving these problems.