In an age of rogue regimes and pre-emptive war, states developing clandestine nuclear programs know better than to leave them in plain sight. Anxious to ward off an American or Israeli attack, Iran, for example, appears to have buried its uranium-enrichment halls under 30 feet of earth and concrete. No doubt, canny proliferators will soon dig even deeper and better-armored holes. But if they dig deeper, we can always go higher: hence the call for the “Rods From God.” More properly known as hypervelocity rod bundles, these weapons would simply be slender solid tungsten cylinders, 20 or 30 feet long...