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 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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