With nations such as Iran protecting their arsenals by burying them deep underground, the Pentagon’s Defense Advanced Research Projects Agency is searching for new types of bunker-buster bombs . The goal is to go beyond the traditional brute-force approach of giant bombs that rely on the force of gravity to smash their way deep into the ground before exploding. During World War II, the Royal Air Force dropped 22,000-pound “Grand Slam” bombs. Eight decades later, the U.S. Air Force has used the