The GBU-57 Massive Ordnance Penetrator (MOP) is an American 30,000-pound (14,000 kg) heavy bunker-buster. Combined with precision guidance, it can target deeply buried and fortified targets such as underground facilities, bunkers, and command centres. The GBU-57 gained widespread attention during its first-ever combat use in June 2025, when US B-2 Spirit stealth bombers dropped multiple GBU-57 bombs on Iranian nuclear facilities.

Images Sourced From: Defense Threat Reduction Agency, U.S. Air Force, DoD, Maxar Technologies

1 History and Development

Development of the GBU-57 began in the early 21st century as a response to a rising concern about US adversaries using hardened underground facilities for the proliferation of weapons of mass destruction (WMDs). The US had bunker buster munitions before, such as the GBU-28 and GBU-37. However, they were insufficient for destroying reinforced underground structures. As a result, the US Air Force (USAF) and the Defense Threat Reduction Agency (DTRA) initiated the development of a new munition with higher penetrating power. [source]

The Air Force Research Laboratory began the development of this weapon in 2004 with Boeing as a main contractor responsible for design, testing, and production. First flight tests were held in 2008 at the White Sands Missile Range, New Mexico. In 2009, Boeing was tasked to integrate the MOP for use with B-2 Spirit stealth bombers, and in 2010, the program fully transitioned under the leadership of the USAF. In 2011, the GBU-57 reached operational capability, with further modifications carried out since then. [source]

According to Chairman of Joint Chiefs of Staff Lt. Gen. Dan Caine, the urgency of the development of the MOP was also guided by intelligence from 2009-2010 revealing Iran’s deeply buried uranium enrichment facility at Fordow. At that time, assessments concluded that the US did not possess the capabilities (an effective ‘bunker buster’ weapon system) to effectively destroy these targets. [source, source]

2 Technical Specifications

The GBU-57 bunker buster has a cylindrical casing made from a high-performance steel alloy that wraps the warhead, designed to be hard and ductile. This is necessary for the MOP to withstand the impact forces, resist deformation, and deliver its kinetic impact to breach layers of geological material. The kinetic energy is responsible for the ground penetration. Thanks to its unique fuzing mechanism, the bomb detonates only after it comes to a stop. It incorporates void sensors that detect changes in material density and delay the detonation until the bomb has entered an open space (e.g. tunnel, underground facility). [source, source]

The GBU-57 has four fixed trapezoidal wings and four steerable lattice (grid) tail fins. The wings generate aerodynamic lift and provide stability during flight.The tail fins provide better control at different speeds and angles of attack to ensure course correction and maneuvering. Additionally, its guidance system keeps track of the MOP’s position, velocity, and altitude to ensure high accuracy. [source, source]

Other technical characteristic include: [source, source, source]

• Length: 6.2 meters (20.5 feet)
• Diameter: 0.8 meters (31.5 inches)
• Body weight: 12,304 kg (27,125 pounds)
• Explosive weight: 341 – 2,423 kg (752 – 5,342 pounds), depending on explosives used
• Penetration capabilities: Officially to 60 meters (200 feet) of earth; 18 meters (60 feet) of reinforced concrete. This can vary based on geological conditions and fortification efforts.
• Guidance: Dual GPS and Inertial Navigation System (INS)

The MOP’s main carriers are the B-2 stealth bomber and, with some modifications, the B-52 Stratofortress. Future deployment will also involve the B-21 Raider. [source]

3 Mission Set

This piece of strategic weaponry succeeds in specialized missions against underground military installations and other critical assets. Some of its main targets include: 

• Underground nuclear facilities: For example uranium enrichment plants, reprocessing facilities, or assembly plants.
• Hardened Command and Control (C2) centres: Destroying underground bunkers can significantly disrupt adversarial communication and leadership functions
• Underground missile silos or storage sites: This includes fortified intercontinental ballistic missile (ICBM) silos and other critical WMD infrastructure. [source, source]

The MOP provides the US with a tool to deny an adversary a refuge within deeply buried installations, thereby enhancing US deterrence and counterproliferation capabilities. 

3.1 Operational Use 

On 22 June 2025, the US decided to strike Iran’s underground facilities under the codename Operation Midnight Hammer. Seven B-2 bombers dropped a total of 14 GBU-57 bunker buster bombs, with 12 targeting the Fordow uranium enrichment plant and two striking the Natanz nuclear facility. [source, source]

While formal bomb damage assessments are ongoing, these attacks seem to have significantly damaged the integrity and operational capability of the facilities, with the Pentagon claiming a setback to Iran’s nuclear program by as much as two years, demonstrating a first successful use of the MOPs against targets that were out of reach for conventional munitions. [source]

3.2 Criticism and Challenges

Despite its effectiveness, the MOP faces multiple challenges. Its cost and immense weight require deployment from specialized aircraft such as the B-2. The limited number of B-2s and GBU-57s puts logistical restraints on its use, limiting it to high-priority targets only.

While the cost of a single bomb is unknown, it is estimated to be in the tens of millions of USD per unit as it features advanced technologies and requires a specialized manufacturing process. Beyond that, one flight hour of a B-2 bomber costs approximately USD $150,000, and there are additional costs for mission planning, maintenance, refuelling, and training that make any use of the MOP a calculated decision. [source]

Additionally, the penetration performance can vary significantly, depending on the geological composition and quality of structure reinforcement. Very dense material or multiple layers of concrete reinforcement can result in less penetration than officially advertised, and complex, multi-chambered underground structures can potentially overwhelm the fuzing mechanisms. This can require the use of multiple MOP’s to “crack up” the ground enough to effectively destroy a target, adding pressure to the already limited stockpile of these units. [source, source]

4 Conclusion

The GBU-57 MOP is a unique military tool designed to address specific challenges coming with adversarial underground structures that are untouchable by conventional bombing methods. By combining massive kinetic energy, precision, and smart void-sensing fuzes, it gives the US an upper hand in dealing with deeply buried hardened targets. The bunker busters used against targets in Iran have gone a long way towards proving their ability to achieve the intended mission. Despite its limited stockpile and logistical challenges, it will remain a strategic asset in the USAF’s arsenal, with further integration expected on the upcoming B-21 Raider aircraft.