Iran’s most heavily fortified nuclear site, Fordo, was built deep inside a mountain to protect it from an attack. Only the U.S. military has the 30,000-pound bomb capable of even reaching it.

The bomb is commonly known as a “bunker buster” because it is designed to destroy deep underground bunkers, or well-buried weapons in highly protected facilities. It is believed to be the only air-delivered weapon that would have a chance of destroying the site.

The bomb has a much thicker steel case and contains a smaller amount of explosives than similarly sized general-purpose bombs. The heavy casings allow the munition to stay intact as it punches through soil, rock or concrete before detonating.

Its size — 20 feet long and 30,000 pounds — means that only the American B-2 stealth bomber can carry it.

Conventional wisdom has been that Israel can’t destroy Fordo on its own. The United States has blocked Israel from getting the bunker buster, and while Israel has fighter jets, it has not developed heavy bombers capable of carrying the weapon.

But Israel can come close by hitting more accessible power generation and transmission plants that help run the facility, which contains Iran’s most advanced centrifuges, military officials said.