Background

The GBU-57 Massive Ordnance Penetrator is the largest conventional weapon in the US arsenal, weighing 30,000 pounds (13,600 kg). Its hardened Eglin-steel casing penetrates up to 200 feet of earth or 60 feet of reinforced concrete before detonating a 5,300-pound payload. Developed by Boeing under a programme accelerated after 2003, the MOP can only be carried by the B-2 Spirit stealth bomber, which holds two simultaneously.

The GBU-57 became central to the Iran conflict when CSIS placed its per-unit cost at approximately $3.5 million within a $3.7 billion total for the first 100 hours of Operation Epic Fury. B-2s flew confirmed sorties against Fordow and Natanz Fuel Enrichment Plant, Iran's most deeply buried nuclear sites. Defence analysts noted early in the campaign that MOP use had not been independently confirmed; only GBU-31 penetrating munitions were initially identified.

The Fordow strikes exposed the central question the MOP was built to answer: whether any conventional bomb can destroy a facility inside mountain rock. The IAEA confirmed structural damage to Natanz entrance buildings but could not verify the underground halls. That ambiguity places the MOP at the threshold of conventional and nuclear coercion.

Common Questions

What is the GBU-57?

The GBU-57 Massive Ordnance Penetrator (MOP) is a 30,000-pound precision-guided bunker-busting bomb, the largest conventional weapon in the US arsenal. It can penetrate up to 200 feet of earth or 60 feet of reinforced concrete before detonating its 5,300-pound payload. Only the B-2 Spirit stealth bomber can carry it.Source: CSIS

How much does the GBU-57 cost?

CSIS estimated the GBU-57 at approximately $3.5 million per unit in 2026, during cost analysis of Operation Epic Fury. Precision munitions dominated the campaign's $3.7 billion price tag for the first 100 hours.Source: CSIS

How deep can the GBU-57 penetrate?

The GBU-57 is rated to penetrate up to 200 feet of earth or 60 feet of reinforced concrete. This capability was designed to reach targets like Fordow, built inside a mountain under metres of rock. Whether it achieved that in combat remains disputed.Source: The War Zone / Army Recognition

Was the GBU-57 used against Iran in 2026?

Defence analysts assessed early in the Iran conflict that GBU-57 use against nuclear facilities had not been independently confirmed; only GBU-31 2,000-pound munitions were initially identified. However, CSIS cost estimates included GBU-57 unit costs in its $3.7 billion tally for Operation Epic Fury's first 100 hours, implying deployment.Source: The War Zone

What is the difference between the GBU-57 and GBU-31?

The GBU-57 weighs 30,000 pounds and penetrates up to 200 feet of earth; the GBU-31 weighs 2,000 pounds and penetrates roughly 1 to 2 metres of reinforced concrete. The GBU-31 was the primary munition confirmed used against Iranian facilities; Fordow and Natanz were built to defeat weapons of that class.Source: Army Recognition

How the World Sees Them

Iran

Iran built Fordow inside a mountain specifically to survive conventional attack, calculating no existing bomb could reach it. The MOP programme and the ambiguity over whether it succeeded represents Iran's core deterrence calculation being tested in real combat.

China

China has studied the MOP programme as a direct threat to its own hardened underground facilities. The Fordow precedent signals that deeply buried infrastructure is now within reach of US conventional strikes.

Israel

Israel lobbied for B-2 access to MOP delivery capability for over a decade, viewing Fordow as the facility that rendered air power options incomplete. The Iran conflict finally provided the strike Tel Aviv could not independently execute.

United States

The US developed the GBU-57 to hold hardened Iranian and North Korean facilities at risk when nuclear options are off the table. Its confirmed deployment against Fordow was the first operational use in any combat theatre.