Countries need to design nuclear devices to adapt with changing defensive technology.  There are a number of technological developments that have made the use of conventional weapons ineffective in combating certain threats. For example, some bunkers are buried so deeply underground that conventional bombs cannot penetrate them. Weapons such as the Robust Nuclear Earth Penetrator (RNEP), currently in development in the United States, would be able to penetrate such bunkers, while leaving no more surface damage than a conventional bomb1. Deployment of a weapon such as the RNEP might prove necessary in order to stop proliferation of nuclear weapons in rogue states, as for example, Iran has built extremely tough bunkers for the purpose of nuclear testing and storage of weapons of mass destruction. Blocking the development of necessary tactical nuclear technologies actually raises the chances of these dangerous states obtaining nuclear weapons. Another instance of tactical nuclear devices proving useful is in the destruction of clandestine biological and chemical weapons factories. Were such facilities destroyed by conventional bombing, some of the materials being manufactured could easily leak into neighbouring population areas, leading to increased casualties. Clearly, in light of these defense innovations, tactical nuclear weapons are an essential addition to a nuclear power's arsenal. 1 Reynolds, Paul. 2003. "Mini-Nukes on US Agenda". BBC News.