Introduction A recent study of laser-assisted mechanical rock tunneling, conducted at the United Aircraft Research Laboratories under Department of Transportation sponsorship, had as a goal the investigation of laser-assisted mechanical rock fracture for use in tunneling machines boring in extremely hard rock. The results of this study, which are reported in Ref. 1, indicate that the use of the laser in a low-power-density mode to preweaken the rock without causing malting of the rock surface is indeed a feasible mode of weakening hard rock, which does allow a reduction in the energy required for subsequent mechanical removal of the rock. However the system studies undertaken in Ref. 1 indicate that the amount of strength reduction achieved by the laser or any other type of heating system would not be sufficient to justify economically the addition of an expensive heating system to a hard rock tunneling machine. One avenue toward employing concentrated heat energy which appears to have a potentially greater efficiency in reducing potentially greater efficiency in reducing the strength of hard rock was identified in the Ref. 1 study. This concept involves using highly concentrated thermal energy to form very narrow melt cuts in the rock. This mode of rock weakening has two advantages relative to the use of the low-power-density heating mode in which rock melting is not allowed to occur. For one thing, the use of an extremely high-power-density energy beam allows penetration of the heat into the rock at substantially higher speeds than the nonmelting case, in which penetration speeds are based on the speed of thermal diffusion into the solid rock. Secondly, there are indications that the deposition of heat at depth in the rock could allow removal of a much greater rock volume than the same amount of heat if it were allowed simply to heat-weaken an entire surface area. Careful use of a very highly concentrated heat tool could allow weakening of rock along specific planes, with subsequent removal of relatively large pieces of rock bounded by these planes.