Some have argued in peer-reviewed journals that even a low yield underground blast would at least shoot fallout through its entry hole (Roman candle-style), contaminate water supplies for centuries and if detonated beneath a highly populated area would lead to tens of thousands of eventual deaths. Others state that it is not possible to even conceive of a missile that could pass more than a four times its own length through reinforced concrete.
Advocates of these earth penetrating "mini-nukes" counter that the lack of current technology does not negate its possible future feasibilty. They go on to say that underground explosions are effectively an order of magnitude more powerful than an air burst due to the increased ability of solids to transmit shock. Even so, say detractors, the inability of these weapons to penetrate past the measured upper limit of 30 times their length in soil, will necessitate yields in the 3-kiloton range which, given the shallow depth, would result in crater formation and the release of fallout -- thus negating their perceived increased safety.