Recently released Russian data on two underground nuclear explosions in an Azgir salt dome allow comparison for the first time with similar U.S. explosions. In both cases, highly tamped explosions were employed to form cavities and, after a period of several years, much smaller explosions were then carried out in these same cavities. The seismic decoupling factor obtained from the Russian tests appears to be a factor of 4 less than is predicted, based on the U.S. experience. However, the Russian data are found to be entirely consistent with smaller scale U.S. experiments with chemical explosives. These results can be explained by the extent of damage in the immediate vicinity of the cavity walls, but it remains unclear why the damage should exist at the Russian site, and not at the U.S. ¿ American Geophysical Union 1993