FAS Public Interest Report - Low-Yield Earth-Penetrating Nuclear Weapons

http://www.fas.org/programs/ssp/nukes/new_nuclear_weapons/loyieldearthpenwpnrpt.html

... the use of any nuclear weapon capable of destroying a buried target that is otherwise immune to conventional attack will necessarily produce enormous numbers of civilian casualties. No earth-burrowing missile can penetrate deep enough into the earth to contain an explosion with a nuclear yield even as small as 1 percent of the 15 kiloton Hiroshima weapon. The explosion simply blows out a massive crater of radioactive dirt, which rains down on the local region with an especially intense and deadly fallout.