Los Alamos National Laboratory (LANL) and its contractors have developed a conceptual design for a directional microborehole drilling system for hard-rock boring. Analytical calculations, simulations, and the results of laboratory testing of critical prototype drilling components have influenced the design. Two reduced-size drilling systems to produce small diameter, 500-ft-Iong, directionally drilled river crossing trajectories are proposed to prove feasibility of the concept: 1) a 2-lI4-in. diameter, early demonstration unit to drill directional ultraslimholes; and 2) a 1-lI8-in. diameter, ultimate design to drill directional microboreholes. Both concepts use versatile, coiled-tubing-deployed, hydraulically-powered drilling assemblies, and a surface platform that includes a tubing injector unit to develop high load insertion (snubbing) of the tubing into the sealed borehole. Surface injection will be used to develop the required bit thrust, and both concepts provide for the use of a commercial, real-time, location and steering system that is readily and routinely adapted for deployment on a coiled-tubing drilling platform. The conceptual drilling platform and its subassemblies are shown in Fig. 1.