The Hess Deep rift valley, at approximately 2° 14' N, 101° 33' W, displays exposures of young, lower crustal and upper mantle rocks formed at the nearby, fast-spreading East Pacific Rise. A seismic refraction experiment was conducted across the Hess Deep rift valley to provide P-wave travel times between sea floor explosions and Ocean-Bottom Seismometers. These travel time data were analyzed using an iterative, damped least-squares, inverse method to produce a velocity model of the subsurface structure. The resulting velocity contrasts were interpreted as lithologies originating at different depths and/or alteration of the preexisting rock units. Petrologic and bathymetric data from previous studies were used, along with the seismic interpretation, to produce a geologic model. The model supports low-angle detachment faulting with serpentinization of peridotite as the preferred mechanism for creating the distribution and exposure of lower crustal and upper mantle rocks within the Hess Deep.