What could we do if we had evidence terrorists were close to acquiring nuclear weapons, or even making their own? How would we defend ourselves from a blast with fireballs hotter than the sun? Today, defiant nations desperately pursue them, while at home, agencies prepare for the worst. National Geographic races from motorcade to helicopter to confidential meeting, as scientists and intelligence experts reveal the hard facts about the worlds most lethal weapons and the unbreakable physical laws that govern them. Find out the actual capacities of a spy satellite, how we can be sure if an underground test is really atomic, and why terrorists might prefer to leave a dirty bomb unexploded.