The scenario seems straight out of a high-stakes action movie: how can you obliterate a mountain made of concrete and granite, and then destroy the protected facility beneath it without resorting to nuclear weaponry? The answer lies in a colossal 30,000-pound bomb, equivalent in weight to a city bus, shaped into a cylinder approximately 20 feet long and 2.5 feet in diameter. Since the outbreak of hostilities between Israel and Iran last week, there’s been intense focus on this formidable weapon: the GBU-57/B. This is the most powerful conventional bomb capable of annihilating underground facilities, and the United States is the sole possessor. Many are wondering if the U.S. will supply this bomb to Israel.

To grasp why the GBU-57/B is significant and why Israel might seek it, one must consider the primary target: Fordo, Iran’s most sophisticated nuclear enrichment site, located 18 miles northeast of Qom. According to a report by the Institute for Science and International Security, Fordo can rapidly produce enough enriched uranium for a nuclear bomb within just a few days. It’s strategically situated deep beneath a mountain, under 260 to 300 feet of rock reinforced with concrete, and shielded by numerous air-defense systems. After Israel’s airstrikes on nuclear sites in Iraq in 1981 and in Syria in 2007, Iran decentralized its nuclear program across several deeply buried sites, beyond the reach of Israel’s 5,000-pound bunker-busting bombs. Reports indicate construction at Fordo began in 2002, although Iran claims it started in 2007, only acknowledging the site’s existence in September 2009.

The designation GBU-57/B begins with “GBU” for “guided bomb unit,” indicating a precision-guided munition, and it’s the 57th design in this series. The second ‘B’ signifies a version update, a common practice in military nomenclature to denote modifications by engineers. Post the 2003 invasion of Iraq, U.S. military engineers evaluated previous bunker strikes using older, smaller GBU models and found they lacked the necessary penetration and inflicted minimal damage. A stronger solution was needed that would adhere to the global norm against nuclear arms—a concept that regards the use of nuclear weapons as ethically repugnant and geopolitically risky due to potential radioactive fallout and diplomatic repercussions. The Massive Ordnance Penetrator (MOP), or GBU-57/B, was developed as a weapon capable of tackling complex missions to neutralize well-protected enemy arsenals without crossing the nuclear threshold.

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Dropped from the B-2 Spirit bomber’s maximum altitude of 50,000 feet, the GBU-57/B employs advanced fins to steer towards its target. While its exact impact speed remains classified, it is known to surpass the speed of sound, Mach 1 (767 mph). This impact delivers kinetic energy equivalent to that of a Boeing 747-400 aircraft landing at 170 mph or an Amtrak Acela train at 120 mph—but concentrated on a much smaller scale. According to a 2012 briefing by the Congressional Research Service, the bomb can penetrate up to 200 feet of concrete or similarly dense bedrock. Following penetration, its 5,300-pound explosive payload detonates.

The design of the GBU-57/B maximizes its penetration capabilities. It features an ogive-shaped nose, reminiscent of a Gothic arch, which is effective in distributing weight and minimizing air resistance. Upon impact, this shape helps distribute the force evenly across the bomb’s steel casing, preventing it from shattering as it drills into the earth. The bomb’s design also boasts high sectional density, meaning it concentrates a significant amount of mass behind a relatively small impact area, enhancing its ability to bore deep into solid rock.

About one-fifth of the bomb’s total 5,342-pound weight is comprised of two types of explosives: 4,590 pounds of AFX-757 and 752 pounds of PBXN-114, both engineered to produce a substantial blast while withstanding the initial impact shock. The bomb’s detonation is controlled by a sophisticated fuse that can be programmed from the cockpit, capable of detecting and counting through layers of rock or concrete to ensure the charge only detonates after reaching the intended depth, delivering an explosive force comparable to several tons of TNT.

Although Israel is suspected of possessing nuclear capabilities, it has adhered to the nuclear non-use norm. To effectively neutralize Fordo without resorting to nuclear weapons, Israel would require the GBU-57/B along with a B-2 Spirit bomber, which is uniquely capable of deploying this giant bomb. Otherwise, Israel might only manage peripheral damage to the site. Given Fordo’s robust defenses, as noted in a Royal United Services Institute piece, even the GBU-57/B might need multiple hits at the same target to breach the facility. Despite rumors of its use in Yemen, the U.S. Air Force clarifies that the GBU-57/B has never been deployed in combat and remains in limited supply, with none so far allocated to Israel or paired with the necessary B-2 bomber.