The Department of Defense is spending $45 million to investigate the feasibility of using ground penetrating bombs carrying nuclear weapons. Since World War II, when the practicality of "ground penetrating" bombs was first demonstrated, work has continued. Modern technology has led to the development of computer controlled fuzes that can have the bomb detonate when it has reached a certain depth, penetrated a certain type of material or gone through a certain number of floors in a bunker. If these weapons already work with conventional explosives, why use nukes? The main reason is that some big time bad guys (North Korea and Libya being two examples) have built huge underground complexes, shielded by concrete, rock and dirt, often to the point where current bunker buster bombs can't do much besides blast the known entrances (and, in the case of the North Koreans, it's been discovered that their underground complexes usually have many secret entrances.) A nuclear bunker buster need only penetrate in the general area of the bunker and then detonate, setting of a local earthquake that would collapse most of the complex. Alas, you still have lots of radioactive material buried down there, and some would spew out the penetration hole. Moreover, anyone who used nuclear weapons again would take a major hit in the world popularity department. The Department of Defense is using computer simulations to see what the effects of such nuclear penetrating bombs would be. The idea is, apparently, to do all the design work and thus make it possible to quickly put such a weapon to use if a truly horrendous situation arose where there was no other alternative.