A U.S. attack on Iran's key underground nuclear facility would not be a guaranteed success, according to a national security expert.

Fordo is Iran’s second nuclear enrichment facility after Natanz, its main facility, which already has been targeted by Israeli airstrikes. The IAEA said Tuesday it believes the strikes have had “direct impacts” on the facility’s underground centrifuge halls.

Iran's most fortified nuclear facility, called Fordo, is buried deep inside a mountain. Only the U.S. has the 30,000-pound bombs — often referred to as "bunker busters" — capable of reaching it. And that option revolves around Missouri's Whiteman Air Force Base.