Research was directed at establishing the technical and economic feasibility of excavation systems based upon the rock-melting (Subterrene) concept. A series of electrically powered, small-diameter prototype melting penetrators was developed and tested. Research activities include optimizing penetrator configurations, designing high-performance heater systems, and improving refractory metals technology. The properties of the glass linings that are automatically formed on the melted holes have been investigated for a variety of rocks and soils. Thermal and fluid-mechanics analyses of the melt flows were conducted with the objective of optimizing penetrator designs. Field tests and demonstrations of the prototype devices continue to be performed in a wide range of rock and soil types. Primary emphasis was placed on the development of a penetrator designed for more economical extraction of geothermal energy and of small-diameter penetrators which can be utilized in support of geothermal energy exploration programs. Optimization of well design, the trade-off of advance rate with operating life, the advantages of using the melt-glass hole casing for well-bore seal-off, rig automation, and the benefits which result from the insensitivity of rock melting to formation temperatures and geologic variations were also studied. Subsystem hardware development was directed toward resolution of critical technical questions related to penetrators for dense rock, debris handling, electrical heater configuration, and establishing penetrator life. Laboratory experiments and field tests provide data for final system design optimizations and indicate proof of applicability of the concept to a geothermal well hole-forming system. A field test unit to form relatively shallow vertical holes for heat flow surveys in support of geothermal exploration studies has been designed, fabricated, and field tested.