The U.S. continues to work on a deep penetrating bomb that uses nuclear explosives to blast deeply buried bunkers. Many scientists believe that a "penetrator" type bomb cannot burrow through more than about 14 meters of rock, or 30 meters of earth. Thus the eagerness to design a small nuclear warhead that would detonate once the penetrator bomb had gone as far as it could. The U.S. already has a conventional penetrator bomb, the 5,000 pound CBU-28. This weapon has been shown to penetrate 30 meters of earth or six meters of concrete, or a combination of the two. But the CBU-28 warhead contains only 630 pounds of explosives. This is enough to destroy any bunker it gets into, but not to smash a bunker that is another 10-20 meters down.
But you could replace the high explosive warhead with a nuclear bomb with the explosive equivalent of one kiloton (or more.) The "kiloton" means roughly equivalent to a thousand tons (two million pounds) of conventional high explosives. The only problem is that the hole the penetrator has just created acts like a chimney when the nuclear bomb goes off, spewing tons of radioactive earth and dust into the atmosphere. It's known from actual underground tests that this occurs. But those tests were with a 100 kiloton bomb detonated 192 meters underground. Computer models have shown that much smaller nuclear bombs (one kiloton or so) might not "spew." But to know for sure there will have to be live tests. And the tests are not just for the "spewing" problem, but also to see if a nuclear weapon can be built to handle the stress of hitting the ground at high speed and then coming to an abrupt halt (in a second or two). Nuclear weapons, which are precision instruments that must operate flawlessly for there to be a nuclear explosion, have been built to take the stress of being fired out of a cannon. For over four decades, the United States built and tested artillery shells that contained nuclear warheads. But a penetrator applies even more stress than encountered by an artillery shell in flight.
Another new area of research is in the penetrators themselves. It's going to take a combination of penetration and nuclear explosives to take down a bunker burned beneath a hundred or more meters of rock. American intelligence agencies have identified (as best they can) some 1,700 deep bunkers world wide. These bunkers can be used to store nuclear, chemical or biological weapons, as well as conventional weapons and headquarters. American bomb designers are asking for $50 million to research the subject over the next few years. With all the unknowns, they may conclude that it's not worth pursuing. That leaves one to deal with these bunkers the old fashioned way; blast their entrances or send in troops and combat engineers to take the bunkers apart, starting from ground level.