Geologic factors that affect the location, construction, and operation of underground installations are primarily structural and petrographic. The energy from a subsurface blast is transmitted outward unequally in a heterogeneous rock. The degree of rock inhomogeneity is in direct ratio to the inability of the mass to resist near a blast the explosive pressure that is superimposed on the pre-existing inherent static stresses.A pattern of rock failure resulting from explosive stresses has been empirically determined (McCutchen). The pressure front of a blast is considered to extend outward as an equal radial stress. Initial and most widespread failure is in tension and tangent to this front; shear planes develop within a smaller inner zone.The size and cross-sectional shape of suitable openings are controlled by the cover: its depth, physical character, planes of inherent weakness, and possible active stresses. Existing openings may not be acceptable.Reconnaissance surveys may be restricted both areally, due to operational factors, and by acceptable rock types. Geologically, site selection involves consideration of the degree of rock deformation, attitude of bedding-foliation, weathering and hydrothermal alteration, inherent active stresses, and water supply requirements.i Part of this paper was presented before the Cordilleran Section, Geological Society ofAmerica, in Berkeley, California, April 16, 1949 as "Some Geologic Aspects of UndergroundConstruction." Views expressed herein are personal and not necessarily those of any govern-ment organization or agency. Sufficient geologic exploration and interpretation to determine the inherent structure, physical characteristics of rock, ground water conditions, a facies change, depth of weathering, and areas and zones of alteration must precede final plant design. This will involve regional studies and detailed surface and subsurface observations at the site. Core drilling, pilot bores, geophysics, and photo-geology may be used to advantage in specific locations.The inherent structure may act either as a "protective barrier" or as an "avenue" transmitting destructive stress toward the opening, depending on its magnitude and location with respect thereto.