Calculations on the cratering and ground motion in a rock medium due to a two-megaton surface burst. The theoretical approach assumes a two-dimensional hydrodynamic model, and it is used to determine the motions involved in the cratering from a large-yield surface burst. Thetechnique is found to work well and to check with experimental observations. It is shown that the primary cause of cratering for such an explosion is not "airslap," as previously suggested, but rather the direct action of the energitic bomb vapors. High-yield surface bursts are therefore less effective in cratering by that portion of the energy that escapes as radiation in the earliest phases of the explosion. The cratering action and ground shock from large-yield explosions is of primary importance to problems of hardening military installations as well as to the peaceful use of nuclear explosions.