The influence of water in silicate rocks in which underground nuclear explosions were detonated has been observed. Explosions in high-water-content rock cause larger cavities to be formed, and in cratering shots higher peak spall velocities are measured when the detonation occurs in a high-water-content rock. A water-boost model is proposed whereby the region around an underground nuclear explosion is treated as a two-component system on release from its peak shock pressure. The region referred to is that in which the rock is melted or just heated and the water contained within is totally or partially vaporized. The release path for the system is calculated from the release paths of the two components weighted by the fractional percentage of each component. The model was tested by making one-dimensional calculations with a computer code designed to calculate the effects of underground explosions on the environment. The calculational results were compared with measurements in rocks with water contents ranging from a fraction of a percent to over 25% by weight. Excellent agreement was obtained for peak stress in the kilobar region, the cavity radius, and the free surface velocity for a cratering event. For close-in measurements of shock-wave time of arrival and peak pressure in the 100-kb region, a phase change in the rock component, similar to that observed to occur in rock shocked to high pressures in excess of 100 kb, was introduced to give good agreement between measurement and calculation in this region. The importance of the rock component phase change further out, in the kilobar region, does not seem to be significant.