The US is concerned that potential enemies and rogue nations are putting their high value assets (weapons of mass destruction, command and control facilities, chemical and biological weapons production facilities) into deep tunnels. With hundreds of feet of rock above them, there is no practical way to destroy them with conventional bombs. Beyond a certain point, each bomb just brings down more rubble from higher slopes and doesn't penetrate farther into the mountain. The solution is to take an existing nuclear bomb (perhaps the B61 or B83) and place it into a new 5,000-pound class penetrating bomb casing. The US eventually wants to use smaller nuclear weapons so that enemies will honestly believe that the US would use them.--Stephen V Cole