The specialized underground nuclear tests conducted at the Nevada Test Site require preshot geologic and geophysical background data for their efficient and successful completion. Large negative gravity anomalies are produced by the density contrast between the Cenozoic sedimentary and volcanic rocks and the more dense older rocks, and gravity surveys have played an important role in determining the thickness of the Cenozoic rocks. In Yucca Flat many estimates of the thickness of the Cenozoic rocks have been made from gravity anomalies. Thickness estimates checked against drill hole data indicate an average density contrast of 0.7 gm/cc, but minor adjustments of the assumed average density contrast are required locally. Thickness estimates of Cenozoic rocks checked in drill holes have an average error of approximately 18 percent. A reconnaissance gravity survey of the Pahute Mesa area north of the Nevada Test Site defined the largest gravity low in the Nevada Test Site area. By application of the maximum density contrast believed reasonable (0.7 gm/cc) to the 63-mgal anomaly, a minimum thickness of 7000 feet of volcanic rocks was estimated to underlie Pahute Mesa. Exploratory drillings for deep testing media was begun after the test site boundary was extended to include the mesa, and cores brought up from a drill hole 13,686 ft deep were still of volcanic rock. Data from core and density logs indicate a density contrast here of only 0.33–0.36 gm/cc. Analysis of the gravity low, assuming this density contrast, indicates a thickness of 16,000 ft of volcanic rocks.