Yucca Flat is a topographic and structural basin in the northeastern part of the Nevada Test Site (NTS) in Nye County, Nevada (fig. 1). Between the years 1951 and 1992, 659 underground nuclear tests took place in Yucca Flat (U.S. Department of Energy, 2000); most were conducted in large, vertical excavations that penetrated alluvium and the underlying Cenozoic volcanic rocks (U.S. Department of Energy, 2000).