This ‘Bunker Buster’ U.S. Bomb Could Cripple Iran’s Nuclear Ambitions

The 30,000-pound Massive Ordnance Penetrator is considered by analysts to be the best shot at getting through to targets buried deep under mountains.

The best shot at knocking out the most fortified part of Iran’s nuclear program comes down to a giant U.S. bomb that has never been used in war.

The latest GBU-57 A/B Massive Ordinance Penetrator bomb can penetrate about 200 feet (61 metres) below the surface before exploding.(Screengrab/YouTube/AP)

The GBU-57—also called the Massive Ordnance Penetrator—is a 30,000-pound behemoth encased in a high-density steel alloy designed to plummet through 200 feet of mountain rock before exploding.

Military analysts said that large bunker buster has the best chance of getting through to such targets as the Fordow uranium-enrichment facility, which Iran buried under a mountain. Its existence has driven speculation that the U.S. could get involved in Israel’s attack.

“This is really what it was designed for,” said Mark Cancian, who matched bombs to targets in the military and later worked at the Pentagon on procurement and budgeting, including for programs like the MOP.

Before bunker busters, the military figured it could turn to nuclear weapons to blast through mountains, but those were seen as unpalatable for political reasons, said Cancian, now a senior adviser at the Center for Strategic and International Studies.

Later, the U.S. worked on a new conventional alternative and spent about $400 million to develop and refine the MOP, he said. The U.S. now has around 20 of the giant explosives, he said, designed to be delivered by B-2 stealth bombers.

“It’s a really specialized weapon for a very specialized set of targets that don’t come up very often,” Cancian said.

Israel on Friday launched a campaign of intelligence operations and hundreds of airstrikes aimed at setting back Iran’s nuclear program and hobbling its regime.

Israel notched direct hits on Iran’s underground centrifuge halls at Natanz, some 140 miles south of Tehran, according to the International Atomic Energy Agency. But it has yet to attack Iran’s other enrichment site, Fordow, in central Iran, near the holy city of Qom.

The U.S., which hasn’t joined Israel in the attacks, began building up its military assets in the region in recent days, including bringing in a second aircraft-carrier group. President Trump, who has pushed for a diplomatic solution all year, has turned more bellicose, suggesting Tuesday on social media that Iranian Supreme Leader Ayatollah Ali Khamenei could be killed and calling for unconditional surrender.

If the U.S. were to get involved, it would make sense for it to take on hardened targets like Fordow and Natanz, said Mick Mulroy, former U.S. deputy assistant secretary of defense for the Middle East. Destroying them could take half a dozen MOPs apiece, he said.

The United Nations atomic-energy chief has warned of safety concerns from attacking nuclear sites, but other nuclear experts say the radiation risks of an attack on Fordow are low.

The International Atomic Energy Agency reported radiological and chemical contamination inside Natanz, which was bombed Friday, but normal radiation outside.

“If anything were to be dropped on Fordow, there is not a risk of radiation contamination from the attack outside of the site,” said Scott Roecker, vice president for nuclear materials security at the Nuclear Threat Initiative think tank.

Israel has a plan for Fordow and the ability to carry it out on its own, a senior Israeli military official said without elaborating. It is also taking a broader view of its mission by attacking Iran’s military leadership and nuclear scientists as well as components of the nuclear program itself.

Ehud Eilam, a former researcher for Israel’s Ministry of Defense, said Israel could send a large number of its own, smaller penetrator bombs to dig their way into Fordow, as Israel did when it killed the Hezbollah leader, Hassan Nasrallah, in a bunker under Beirut. It could also try a risky commando raid or more-covert means such as cyberattacks and targeted killings, he said.

An MOP dropped by a B-2 bomber could be simpler and better.

“The approach with the highest confidence of success would be a U.S. strike,” said William Wechsler, who was deputy assistant defense secretary for special operations under President Barack Obama.

Write to Benoit Faucon at benoit.faucon@wsj.com