The best chance of destroying the most entrenched part of Iran's nuclear program comes down to a giant American bomb that has never been used in war.

The GBU-57 - also called the Massive Ordnance Penetrator (MOP) - is a bomb weighing 30,000 pounds (13.6 tons) encased in a high-density steel alloy designed to drop through 200 feet of mountain rock before exploding.

Military analysts say this bomb has the best chance of reaching targets such as the Fordow uranium enrichment facility, which Iran has buried under the mountain. Its existence has sparked speculation that the U.S. could join Israel in the attack.

"That (blowing up such sites) is really what (the bomb) was designed to do," said Mark Kanchan, who handled bombs in the Army and later worked in Pentagon supply and budgeting, including for programs such as the MOP.

Before bunkers came along, the military thought it could turn to nuclear weapons to blow up mountains, but they were deemed unacceptable for political reasons, said Kanchan, who is now a senior adviser at the Center for Strategic and International Studies.  

The U.S. later worked on a new conventional alternative and spent about $400 million to develop and refine the MOP, he said. He said the U.S. now has about 20 of the giant explosives designed to be carried 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," the expert said.

On June 13, Israel launched a campaign of reconnaissance operations and hundreds of airstrikes aimed at deterring Iran's nuclear program and thwarting its regime.

According to the International Atomic Energy Agency (IAEA), Israel carried out direct strikes on Iran's underground centrifuges at Natanz, about 140 miles south of Tehran. But it has not yet attacked Iran's other uranium enrichment site, Fordo, in central Iran, near the holy city of Qom.

The U.S., which has not joined Israel in the attacks, has begun increasing its military assets in the region in recent days, including bringing in a second group of aircraft carriers. President Trump, who has pushed for a diplomatic solution all year, has become more bellicose, suggesting on social media on June 17 that Iran's supreme leader Ayatollah Ali Khamenei could be assassinated and calling for unconditional surrender.

If the United States were to intervene, it would be logical to take action against fortified targets such as Fordow and Natanz, said Mick Mulroy, former U.S. undersecretary of defense for Near East affairs. Destroying them could take half a dozen MOPs, he said.

The head of the United Nations atomic energy division has warned of safety concerns over attacks on nuclear sites. Other nuclear experts say the radiation risks of an attack on Fordow are small.

The IAEA reported radiological and chemical contamination at Natanz, which was bombed on June 13. Radiation outside, however, remained at normal levels.

"If something is dropped on Fordow, there is no risk of radiation contamination from an attack outside the site," says Scott Rooker, vice president for nuclear materials security at the think tank Nuclear Threat Initiative.

Israel has a plan for Fordo and is capable of carrying it out on its own, a senior Israeli military official said, without elaborating. Israel is also taking a broader view of its mission, attacking Iran's military leadership and nuclear scientists, as well as components of the nuclear program itself.

Ehud Eilam, a former researcher at the Israeli Defense Ministry, said Israel could send a large number of its own, smaller, penetrating bombs to dig into Fordow, as it did when it killed Hezbollah leader Hassan Nasrallah in a bunker outside Beirut. Israel could also try risky commando incursions or more covert means such as cyberattacks and targeted assassinations.

Dropping an MOP from a B-2 bomber would be simpler and better. | BGNES

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Benoit Faucon and Andrew Dowell, Wall Street Journal