Proponents of a new generation of low-yield nuclear earth-penetrating weapons (EPWs), such as modified versions of the B61-11 currently in the US stockpile, claim that such weapons could be used against deeply buried and hardened underground bunkers with "minimal collateral damage." Even a very low-yield nuclear EPW exploded in or near an urban environment will, however, cause radioactive dirt and debris and other radioactive material to fall out over several square kilometers. A nuclear EPW with a yield less than one-tenth of that of the nuclear weapon used on Hiroshima or Nagasaki could result in fatal doses of radiation to tens of thousands of victims. Biological and chemical agents stored in targeted bunkers may be dispersed into the atmosphere without being destroyed by an EPW, potentially injuring or killing unprotected civilians. The number of casualties from a nuclear EPW attack would depend on the location of the target, the density of the surrounding population, the extent of debris dispersal, and the possibility of escape or evacuation. In addition to the acute and long-term medical consequences, use of nuclear weapons would weaken existing restraints against further proliferation or use of nuclear weapons and would cross a threshold that has been maintained since 1945, when the United States detonated the first nuclear weapons over Hiroshima and Nagasaki.