You guys aren't that smart.
The pentagon has been pressuring at least three presidents to concede to the option that at some point our conventional bunker busters won't be that effective as the bad guys build deeper holes. It's rumored that in the battle in Tora Bora in Afganistan several high ranking Pentagon officials actually tabled the option that if we knew where Osama was, and we coulnd't reach him with a IU 2000, then we should consider using a low yield bunker buster to get him.
When one mentions nuclear weapons, the common vision is mushroom clouds and dramatic night explosions leveling cities for miles.
The reality of the situation is not the case. The U.S. administration is simply speaking a bit of code to the Iranians that if they choose to move the enrichment programs under-ground, it doesn't matter how deep they go because if we decide to take the facilities out we might not limit ourselves to conventional weapons.
Note I'm not offering my opinion on the Iranian ordeal, but simply translating what the administration is saying.