The 30,000-pound GBU-57, also known as the MOP, played a key role in Operation Midnight Hammer last month. Air Force B-2 stealth bombers dropped 12 MOP bombs on two different ventilation shafts at Fordow. Another two were used against the Natanz site.

The technology is designed to be capable of attacking underground targets. It can reportedly hit locations hundreds of feet below ground level.

“MOP is a large, GPS-guided, penetrating weapon with the ability to attack deeply-buried and hardened bunkers and tunnels. The warhead case is made from a special high‑performance steel alloy and its design allows for a large explosive payload while maintaining the integrity of the penetrator case during impact,” according to an Air Force description of the system.

The Defense Threat Reduction Agency was heavily involved in testing, modeling and simulation of the system for many years prior to Midnight Hammer, in partnership with the Air Force.

“What we do try to do is test [technologies] in what we call a ‘threat representative environment.’ And in this case, we built a test site to test the munitions against, in collaboration with the Air Force and DTRA’s test organization, to try to ascertain the effects that the MOP would have in certain environments. We’ve continued to do tests over time to then determine what those effects are, and then we use that information to support our modeling and simulation programs. Those models that we’ve built include the weapons effects that we saw during all of the testing events, and include a number of other factors that our experts have brought to bear. That model is also part of that targeting and weaponeering support that we talked about [with regard to Midnight Hammer]. So in addition to the threat representative testing that we did, where we were able to see how does the MOP act in certain situations and certain geographies and architectures, if you will, we also then use that information to support our further modeling and simulation to lead to our best targeting analysis to support those decision makers,” a senior defense official told reporters Thursday during a background call.

U.S. intelligence agencies are still working to complete a final battle damage assessment to better understand the impact of last month’s strikes on Iran’s nuclear sites.

“We don’t conduct the BDA here, but we eagerly anticipate the intelligence community completing BDA on this so that we can assess the models vice what actually happened according to their analysis, and then take a look at how accurate the projections were, so we can use information there to improve our modeling output and our targeting decision support packages that we put together. We also will be able to assess whether or not the weapon performed as planned, according to the BDA,” the senior defense official said.

“Then that information may go into future iterations of the technology,” they added. “We will take this information and determine did things work the way that we wanted them to, in which case, how can we continue to improve upon it? Or did things not work exactly as planned, and how can we fix that so that in the future our next-generation capabilities work that much better? We don’t have that information yet, but we look forward to receiving it so that it can inform our next investments in this arena.”

Notably, MOP fuzes can be programmed.

The bomb is “comprised of steel, explosive and a fuze, programmed bespokely [for] each weapon to achieve a particular effect inside the target. Each weapon had a unique desired impact, angle, arrival, final heading and a fuze setting. The fuze is effectively what tells the bomb when to function. A longer delay in a fuze, the deeper the weapon will penetrate and drive into the target,” Chairman of the Joint Chiefs of Staff Gen. Dan Caine told reporters last month during a press briefing about Midnight Hammer.

The Air Force is pursuing a MOP modification program to integrate a “smart fuze” capability into the weapon. The so-called Large Penetrator Smart Fuze is intended to provide “increased probability of kill” against hard and deeply buried targets “by mitigating the risk of target intelligence uncertainty,” according to a report from the Pentagon’s director of operational test and evaluation.

On Thursday’s call, the senior defense official declined to say whether the MOPs used in Midnight Hammer were equipped with the smart fuze capability.

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The 30,000-pound GBU-57 bombs, also known as MOPs, used in this assault were designed to attack deep underground targets in locations that are hundreds of feet below ground level. 

Caine and Defense Secretary Pete Hegseth briefed reporters at the Pentagon Thursday regarding the military’s planning and execution of the U.S. airstrikes on Iranian nuclear facilities, which marked the first-ever operational employment of the MOP weapon.

Early testing of the MOP began roughly two decades ago under a technology demonstration effort led by the Defense Threat Reduction Agency before being transitioned to the Air Force. Super computers were used for modeling and simulation during the development of the weapon, Caine noted during Thursday’s briefing. The chairman said he met yesterday with two DTRA officers who “spent their life’s work” enabling and demonstrating the complex bombs.

The U.S. used these so-called bunker-buster bombs on two nuclear facilities — in Fordow and Natanz — last weekend. Caine noted that the U.S. targeted ventilation shafts at Iran’s nuclear facility in Fordow.

The MOP is “comprised of steel, explosive and a fuze programmed bespokely [for] each weapon to achieve a particular effect inside the target. Each weapon had a unique desired impact, angle, arrival, final heading and a fuze setting. The fuze is effectively what tells the bomb when to function. A longer delay in a fuze, the deeper the weapon will penetrate and drive into the target,” he explained.

The U.S. military decided to strike two of the ventilation shafts at Fordow as the primary points of entry into the underground facility. In the days preceding the attack, the Iranians attempted to cover the shafts with concrete to try to thwart an attack, according to Caine.

“I won’t share the specific dimensions of the concrete cap. But you should know that we know what the dimensions of those concrete caps were. The planners had to account for this. They accounted for everything. The cap was forcibly removed by the first weapon and the main shaft was uncovered. Weapons two, three, four  [and] five were tasked to enter the main shaft, move down into the complex at greater than 1000 feet per second and explode in the mission space,” he said. “There were six on each side. Weapons number six was designed as a flex weapon to allow us to cover if one of the preceding jets or one of the preceding weapons did not work.”

Two additional MOPs were used on Natanz. A total of 14 were dropped during Midnight Hammer.

During a previous briefing on Sunday, Caine said that early battle damage assessments suggested that the “massive ordnance penetrator” bombs dropped by U.S. Air Force B-2 Spirit stealth bombers severely damaged Iran’s nuclear arsenal.

​​On Thursday, he said all of the MOPs used against Fordow went exactly where they were intended to go. 

“A bomb has three effects that causes damage: blast, fragmentation and overpressure. In this case, the primary kill mechanism in the mission space was a mix of overpressure and blast ripping through the open tunnels and destroying critical hardware. The majority of the damage we assess, based on our extensive modeling, was a blast layer combined with the impulse extending from the shock,” he told reporters.

Hegseth criticized recent reporting about an intelligence assessment from the Defense Intelligence Agency in the wake of the attacks that suggested the airstrikes might not have damaged Iran’s nuclear program as much as senior Trump administration officials have claimed. Hegseth told reporters that there was “low confidence in this particular report.” 

Caine said the intelligence community is in charge of the battle damage assessments.

“But here’s what we know following the attacks and the strikes on Fordow. First, that the weapons were built, tested and loaded properly. Two, the weapons were released on speed and on parameters. Three, the weapons all guided to their intended targets and to their intended aim points. Four, the weapons function as designed, meaning they exploded. We know this through other means, intelligence means that we have,” Caine said. “We were visibly able to see them. And we know that the trailing jets saw the first weapons function, and the pilot stated, quote, ‘this was the brightest explosion that I’ve ever seen. It literally looked like daylight’” even though the attacks occurred in darkness.

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The comments by Vice Adm. Brad Cooper — the current deputy commander of Centcom, who’s been nominated for the top job and promotion to four-star — came just a couple of days after U.S. attacks on Iran’s nuclear infrastructure that featured the first-ever combat employment of “massive ordnance penetrator” bombs.

The Air Force dropped 14 of the so-called MOP weapons from B-2 Spirit stealth bombers during the mission, dubbed Operation Midnight Hammer.

It’s unclear how many MOPs or other so-called bunker-buster weapons the Pentagon still has in its arsenal in the wake of the operation. The Defense Department typically does not publicly disclose specific numbers for its munition stockpiles.

“As we’ve seen throughout the region, groups are going underground, [such as] Hamas, Hezbollah, the Houthis,” Cooper told members of the Senate Armed Services Committee during his confirmation hearing Tuesday. “This is a serious issue that we will have to look at in the future.”

Nation-state and non-state actors have built bunkers, tunnels and other underground facilities to make their personnel and systems more difficult to locate and target.

“I think in the Central Command, and I think we would have to anticipate in the future, globally, you’re going to see threats begin to go underground, whether we’re talking about Hamas, Hezbollah, the Houthis, the Iranians, other adversaries are clearly watching and see where they can gain advantage. In my current capacity, I have visited on multiple occasions the subterranean commando unit in Israel that goes after this problem set. I think, as we look to the future, and if confirmed, I think we need to focus on two areas — sensors and munitions. And if confirmed, I would advocate for both of those,” Cooper said.

Lawmakers and Pentagon officials in recent years have been beating the drum about the need to increase U.S. production of a variety of munitions and other systems as observers have watched forces expend large numbers of missiles and drones in places like Ukraine and the Middle East.

Cooper on Tuesday said he welcomed ideas like the FORGED Act and other measures that could help the Defense Department cut through red tape and bring new technologies into its arsenal.

Another concern raised by lawmakers during the hearing was the growing threat posed by adversaries’ unmanned aerial systems. American troops have come under attack from enemy drones in recent years, including at Tower 22 in Jordan. The weapons have also played a huge role in the Ukraine-Russia war and the recent Israel-Iran war.

Counter-drone capabilities are in high demand, especially in places like the Centcom region.

“I do agree that the nature and the character of warfare is changing before our very eyes, and this is why I think the important work of this committee, whether it’s the FORGED Act or anything associated with it, where you can accelerate the delivery of counter-UAS systems or other warfighting tools into the hands of the warfighters, forward — those are all value added and needed imminently,” Cooper said.

“If I look back specifically toward the Tower 22 incident in the ensuing now 17 or 18 months, we’ve made considerable improvements across the board — layered defense, employing both kinetic capability and non-kinetic capability. We really are leaps and bounds ahead of where we were before. Having said that, I would never be satisfied that we have the maximum readiness. I’ll never be satisfied that we have enough to protect our men and women in uniform. And if confirmed, I would focus on this every single day,” he added.

During his previous assignment as commander of Naval Forces Central Command and 5th Fleet, Cooper oversaw Task Force 59, which focuses on combining AI, uncrewed systems — including commercially owned platforms — and other digital and communications tools to boost the command’s intelligence, surveillance and reconnaissance capabilities in the Middle East.

In written responses to lawmakers’ advance policy questions ahead of his confirmation hearing, Cooper said that, if confirmed as Centcom commander, he would launch new initiatives to advance U.S. military “overmatch” through the employment of cutting-edge technologies, including AI-enabled unmanned platforms and digital integration.

“In my own experience, having commanded the Navy’s first unmanned and artificial intelligence task force, I’m very familiar with the capabilities that exist in America’s elite tech sector. I believe that we need to leverage that tech sector to maximum capability and deliver capability in the very near term, because we could do more,” he told lawmakers at Tuesday’s hearing.

Cooper’s selection to command Centcom is unlikely to face major political opposition in the Senate, and his nomination is expected to be confirmed.

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The event marked the first-ever operational employment of the weapon, Secretary of Defense Pete Hegseth confirmed Sunday during a press briefing at the Pentagon.

The 30,000-pound GBU-57 bomb, also known as the MOP, is designed to be capable of attacking underground targets. It can reportedly hit locations hundreds of feet below ground level.

“MOP is a large, GPS-guided, penetrating weapon with the ability to attack deeply-buried and hardened bunkers and tunnels. The warhead case is made from a special high‑performance steel alloy and its design allows for a large explosive payload while maintaining the integrity of the penetrator case during impact,” according to an Air Force description of the system.

Due to the bomb’s size and design, it can only be carried by the B-2 bomber. Each B-2 can carry two GBU-57s.

Early testing of the MOP began about 20 years ago under a technology demonstration effort led by the Defense Threat Reduction Agency before the technology was transitioned to an Air Force “quick reaction capability program,” according to a service fact sheet. Boeing was contracted in 2009 for MOP-aircraft integration efforts. In 2017, the MOP transitioned to an Air Force program of record, according to a report by the Pentagon’s director of operational test and evaluation.

Notably, in fiscal 2024, the Air Force conducted two full-scale tests to “verify fixes to a B-2 integration issue,” according to a DOT&E annual report.

The Air Force has been pursuing a MOP modification program to integrate a “smart fuze” capability into the weapon. The so-called Large Penetrator Smart Fuze is intended to provide “increased probability of kill” against hard and deeply buried targets “by mitigating the risk of target intelligence uncertainty,” according to the DOT&E report. Defense officials on Sunday did not say whether the MOPs used in Midnight Hammer were equipped with the smart fuze capability.

Seven B-2s and a variety of other assets were involved in Midnight Hammer, which included U.S. attacks on Fordow, Natanz and Esfahan in Iran, Gen. Dan Caine, chairman of the Joint Chiefs of Staff, told reporters at Sunday’s press briefing.

Hegseth said the aim of the mission, which was ordered by President Donald Trump, was to “destroy or severely degrade” Iran’s nuclear program. Western officials were concerned that the Iranian regime could use the country’s nuclear materials and know-how to build weapons of mass destruction.

“At approximately 6:40 p.m. Eastern Standard Time, 2:10 a.m. Iran time, the lead B-2 dropped two GBU-57 Massive Ordnance Penetrator weapons on the first of several aim points at Fordow,” Caine said. “The remaining bombers then hit their targets as well, with a total of 14 MOPs dropped against two nuclear target areas. All three Iranian nuclear infrastructure targets were struck between 6:40 p.m. and 7:05 p.m. Eastern time … with the Tomahawk missiles [launched from a submarine] being the last to strike at Esfahan to ensure we retain the element of surprise throughout the operation. Following weapons release, the Midnight Hammer strike package exited Iranian airspace, and the package began its return home.”

Fighter jets went in ahead of the bombers to guard against enemy fighter aircraft and surface-to-air missiles. The Defense Department is unaware of any shots being fired at the U.S. military aircraft on their way in or out of Iran, Caine noted.

About 75 precision guided weapons were employed during the operation, including the 14 MOPs, according to Caine.

Dozens of air refueling tankers were also involved in the mission, he noted, as well as intelligence, surveillance and reconnaissance aircraft, and hundreds of maintenance and operational personnel.

On Sunday evening, Trump posted on Truth Social that the B-2 pilots who took part in Midnight Hammer had safely returned to their base in Missouri. The mission from Whiteman Air Force Base to Iran and back lasted about 36 hours, with the help of aerial refueling, according to U.S. Strategic Command.

Senior U.S. officials touted the operation as a success.

“The damage to the Nuclear sites in Iran is said to be ‘monumental.’ The hits were hard and accurate. Great skill was shown by our military,” Trump said in a Truth Social post.

Hegseth described the mission as an “incredible and overwhelming success.”

Caine noted that a final battle damage assessment will take “some time” to complete. However, initial battle damage assessments “indicate that all three sites sustained extremely severe damage and destruction,” he told reporters.

Officials in the Trump administration over the weekend said they now hope to have a negotiated settlement to the conflict. They were waiting to see how Iran would respond.

On Monday, Iran launched missile attacks at a U.S. military base in Qatar.

“I can confirm that al Udeid Air Base was attacked by short-range and medium-range ballistic missiles originating from Iran today. At this time, there are no reports of U.S. casualties. We are monitoring this situation closely and will provide more information as it becomes available,” a U.S. defense official said in a statement.

Updated on June 23, 2025, at 2 PM: This story was updated to include comment from a U.S. defense official about an Iranian missile attack on al Udeid Air Base on Monday.

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