The invention described herein was made in the course of, or under, a contract with the U. S. ATOMIC ENERGY COMMISSION. It relates ... to a method and apparatus for drilling, tunneling and shaft-sinking in rock with particular advantage at hitherto inaccessible depths. The present invention uses the basic apparatus and method disclosed in U.S. Pat. No. 3,357,505 and in Los Alamos Scientific Laboratory of the University of California Report No. LA-3243 (1965) entitled Rock Melting as a Drilling Technique." In the existing rock melting devices of the prior art, a major difficulty which limited performance was that of delivering a sufficiently large heat flux to the melting face of the drill or penetrator. The development of the heat pipe alleviates this problem in that the use of heat pipes enables the transfer of heat energy from a compact heat source to the extended melting surface of the penetrator at rates high enough to maintain the surface above the melting temperature of the rock. The extrapolation of a mechanism useful for forming large holes in the earth in accordance with the present invention uses the combination of a refractory rock-melting tool, an in situ heat source preferably a small nuclear reactor and an exceedingly efficient heat transfer mechanism such as a system of heat pipes to convey heat from the source to the walls of the drilling tool.