The U.S. Air Force has backed off on building a nuclear deep penetration bomb, and is instead going with one using conventional explosives. The new bomb, the Massive Ordnance Penetrator (MOP), will weight 15 tons and be carried internally by the B-2 or B-52 bombers. The MOP will be guided, by GPS (as well as other methods as a backup.) First test models are to be available in 2005. Operational characteristics (how deep the bomb can go in different types of material) has not been announced. And probably wont be, as you would want to keep that data secret lest the bad guys dig too deep for MOP to reach them.