The U.S. is retrofitting its B-2 Stealth bombers with massive bunker-buster bombs - a move that could be a prelude to an attack on Iran and its nuclear facilities. Iran has refused to comply with international demands that it stop its nuclear weapons programs. Experts have noted that a U.S. or Israeli strike on Iran's nuclear program could be difficult due to the large number of installations - some of which are buried deep underground in hardened bunkers.