Underground nuclear explosions produce cavities into which the overburden may collapse, forming a rubble-filled chimney. The particle statistics, especially mean surface-volume diameter, are important to various potential applications of nuclear explosives. The available rubble statistics are rather limited, even with the best data obtained from post-shot exploration of the Hardhat event. Two sets of Hardhat data are presented: one based on photographs of relatively undisturbed rubble, and another obtained from visual estimates of what would be obtained by screening. The method of obtaining particle statistics must be consistent with the intended use of the statistics, because the method of measurement, such as handling for sieving, affects the statistics. It has long been established that crushing and grinding (which occur during chimney collapse) force particle statistics toward the lognormal frequency distribution. Both sets of Hardhat data fit the lognormal distribution in a satisfactory manner. The volume-surface mean particle diameter for the undisturbed Hardhat rubble is 1.33 ft, and the estimate for the volume-surface mean particle diameter obtained by screening is 0.433 ft. Additional statistics from the field plus a better understanding of rock-fracture phenomena are required to develop statistical relations for use in prediction and control of underground nuclear explosion effects.