This regional gravity study, based on an irregular 2-km data grid, was conducted during the past few years at Yucca Mountain, southern Nye County, Nevada, as part of a program to locate a suitable repository for high-level nuclear waste. The candidate repository site lies within tuff composing the northern part of Yucca Mountain, a relatively coherent block of rocks within a stratigraphically and structurally complex region. This study explores and interprets the three-dimensional structure and geometry of the rocks surrounding the candidate site.