The U.S. Air Force’s 30,000-lb (≈13.6 t) GBU-57/B Massive Ordnance Penetrator (MOP) is the world’s heaviest precision-guided conventional bomb, purpose-built to smash bunkers buried far deeper than earlier weapons could reach. In June 2025, B-2 Spirit stealth bombers reportedly dropped MOPs in combat for the first time against Iran’s Fordow, Natanz, and Isfahan nuclear complexes. Initial satellite reads and Pentagon leaks indicate that Fordow’s main access tunnels have collapsed, while Natanz and Isfahan suffered fires and blocked entrances. Because MOP can drill through roughly 60 m (≈200 ft) of 5 000 psi concrete or 80–90 m of granite before detonating, Iran’s decision to bury key facilities 80-plus metres underground no longer guarantees immunity. Tehran could dig even deeper, but the engineering costs, construction signatures, and operational bottlenecks would soar—just as the United States continues upgrading the MOP’s smart fuse, follow-through tactics, and potential hypersonic successors.
1. The GBU-57/B in plain numbers
| Attribute | Figure (US units) | Figure (metric) |
|---|---|---|
| Weight | 30,000 lb | 13.6 t |
| Length | 20.5 ft | 6.2 m |
| Diameter | 31.5 in | 0.8 m |
| Penetration | ≈200 ft of 5 000 psi concrete / 60 m hard rock | – |
| Guidance | MIL-spec GPS + INS | CEP only “a few metres” |
| Delivery | Only B-2 can carry two MOPs; B-52 tested but not operational | – |
2. June 2025: first combat drop of the MOP
“In June 2025, B-2 stealth bombers launched from Guam and the U.S. mainland, refuelled twice over the Indian Ocean, and struck Fordow, Natanz, and Isfahan with MOPs and supporting cruise- and JDAM-class munitions.”
Early satellite imagery (Planet Labs) shows scorched terrain and collapsed adits at Fordow, while Iranian media conceded “temporary shutdowns” at Isfahan for safety checks. The IAEA said it is still validating on-site radiation readings.
3. MOP “hard-core” specifications (translated)
Weight & form-factor
The MOP weighs 30,000 lb, measures roughly 20 ft 6 in in length, and 31.5 in in diameter, making it the heaviest precision-guided bomb ever fielded by the U.S. Air Force.
Penetration power
U.S. military and think-tank testing indicate that a single MOP can breach more than 60 m of re-bar-reinforced concrete or about 200 ft of competent hard rock before exploding at a programmed depth.
Guidance
The weapon fuses encrypted GPS with an inertial navigation system, yielding a circular-error probable of only a few metres—enough to punch precisely at mountain slopes, tunnel mouths, or ventilation shafts.
Launch platform
At present only the B-2 Spirit can carry two MOPs internally; B-52H testing proved feasible but was shelved over survivability concerns.
4. Target list and assessed damage
| Facility | Location & role | Main ordnance | Initial damage read-out |
|---|---|---|---|
| Fordow Fuel-Enrichment Plant | Qom province, ≈90 km S of Tehran; 2 000+ IR-1/-6 centrifuges, 20–60 % U-235 | 6 × GBU-57 | Mountain collapse, power & ventilation shafts destroyed |
| Natanz Fuel-Enrichment & Assembly | Isfahan desert plateau; ≥10 000 centrifuges + new spiral tunnel | ~30 × TLAM Block IV / IV+ plus follow-on JDAMs | Surface halls burned; tunnel portals sealed |
| Isfahan Nuclear Tech Centre (UCF + FFEP) | NE outskirts of Isfahan; yellow-cake conversion & research fuel | TLAMs + F-35 precision bombs | Conversion workshop fires; production paused |
(Damage figures are from preliminary commercial-satellite analysis and anonymous Pentagon briefings quoted by the Los Angeles Times; the IAEA has not yet published final readings.)
5. Why Fordow was the “bull’s-eye”
Depth challenge – Fordow’s main halls lie ≈80–90 m under granite, long advertised by Iran as “impossible to crack with conventional weapons.”
Strike tactic – Each B-2 released two MOPs in quick sequence: the first punches a pilot hole, the second rides the same path for deeper reach, reducing Iranian reaction time.
Result – Planet Labs imagery shows collapsed portals and extensive burn scars; U.S. officials claim the “core is disabled.”
6. Natanz: heart of Iran’s centrifuge fleet
Complex layout – Beyond the legacy halls, Iran has excavated a deeper spiral tunnel intended for sixth-generation centrifuges, with modelling suggesting depths of 78-145 m.
Wave-form – Some 30 Block IV Tomahawks launched from subs in the Gulf first softened surface defences, followed by air-dropped JDAMs.
Damage – Multiple centrifuge workshops caught fire; the IAEA reports no abnormal radiation leaks so far.
7. Isfahan: key fuel-cycle node
Facility mix – Yellow-cake conversion (UCF), fuel-plate plant, and research-reactor fuel fabrication lines.
Strike highlights – GPS-guided bombs surgically crippled the conversion line, interrupting Iran’s domestic fuel loop for months, according to U.S. statements.
8. How deep is “too deep”? — Major Iranian sites vs MOP
| Site | Estimated depth / structure | MOP kill-probability | Comment |
|---|---|---|---|
| Arak IR-40 | Surface; 1–2 m RC dome | High via conventional bombs | Nuclear-grade heavy-water reactor |
| Bushehr NPP | Surface PWR, 1.7–2 m dome | Moderate (MOP not essential) | Civil nuclear power |
| Tehran Research Reactor | Surface, 9.6 m pool | High | 5 MW research reactor |
| Karaj centrifuge plant | Light steel sheds | High (MOP unnecessary) | Component factory |
| Parchin HE tunnels | 30–40 m into hillside | Very High | Weaponisation R&D |
| Saghand mine & Ardakan mill | Mostly surface | Low priority | Uranium feed |
| Lavizan-3 tunnels | 40–50 m | High | Underground workshop |
| Natanz old FEP | 40–50 m RC caverns | High; partly hit 21 Jun | – |
| Natanz new spiral | 78–145 m | 80 m zone vulnerable; >120 m riskier | – |
| Isfahan new Line 4 | Depth unverified | TBD | Under construction |
(Depths compiled from CSIS, ISIS-Online & AP imagery analysis.)
9. Why Iran’s depth and fortification still lose to the MOP
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Penetration redundancy & kinetic “over-match” – A Mach-1, 30,000-lb steel dart concentrates ≈8–9 × 10⁸ J of energy, double the margin needed for 80-m granite.
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Smart cavity-sensing fuse – Post-2024 lots incorporate a void-detecting delay fuse that “hears” the target hollow and triggers milliseconds later for maximum cave-in
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Follow-through strike – CSIS notes the B-2’s < 3 m aim-point repeatability, enabling two MOPs down the same hole for another 15-20 m of reach.
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Indirect weak-points – Even partial penetration can choke air shafts or destroy switch-yards, causing high-speed centrifuges to self-destruct.
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Defender’s dilemma – Digging beyond 120 m demands taller peaks or new shafts—work that commercial satellites spot instantly—while ultra-high-strength concrete pours crack more readily under shock.
10. B-2 + global tanker web = “Remote lock-picker”
Stealth ingress – The B-2’s radar cross-section is famously “golf-ball sized,” letting it slip past S-300 radar nets.
Global reach – A sortie from Missouri or Guam requires only two KC-46A refuellings to reach Iran, obviating forward bases.
Seamless carriage – The MOP fits entirely inside the B-2’s bomb bays, preserving stealth and avoiding drag from external stores.
11. Recent GBU-57 upgrades (2023-25)
| Year | Upgrade | Effect |
|---|---|---|
| 2023 | Fragmentation-liner tweak | Wider controlled crack network after detonation |
| 2024 | Smart cavity-sensing fuse | Precise burst at target node |
| 2025 | Strengthened B-2 mounting lugs & new umbilical cables | Eliminated false-fault indications |
12. MOP vs other bunker-busters
| Weapon | Weight | Penetration | Users | Enough to kill Fordow? |
|---|---|---|---|---|
| GBU-28 | 5 000 lb | < 30 m | U.S., Israel | ✗ |
| BLU-109 | 2 000 lb | ~6 m | Multi-national | ✗ |
| GBU-57 | 30 000 lb | 60–90 m+ | United States | ✓ |
Israel’s current GBU-28/BLU-109 stocks cannot crack Fordow; the MOP is tailor-made for that target.
13. Could Iran simply dig to 150 m+?
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Engineering & cost hurdle – Drill/vent cycles slow sharply in 120–180 MPa granite; Iran would need expensive imported TBMs and explosives.
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Ventilation & heat – Centrifuges demand dust-free, isothermal air; at >120 m depth, vertical shafts become mandatory, creating new aim-points.
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Power & cooling logistics – Longer cables and chilled-water lines are easier to sabotage.
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Construction signatures – Massive spoil piles, concrete convoys and heavy-haul tracks are blatant in 30 cm-resolution imagery.
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U.S. counter-tech – The MOP keeps evolving, and hypersonic kinetic-glide or low-yield B61-13 nuclear penetrators loom as next-wave options.
14. U.S. escalation ladder (illustrative)
| Category | Possible tool | Status |
|---|---|---|
| Enhanced conventional | Heavier MOP variant, B-52 carriage, multi-aim-point salvos | FY-24 development completed |
| Tactical nuclear | B61-13 earth-penetrator (dial-a-yield up to 250 kt) | Politically contentious |
| Hypersonic kinetic | LRHW/CPS, post-ARRW concepts | In prototype testing |
| Non-kinetic | Cyber power-grid hacks, insider sabotage | Proven in past (e.g., Stuxnet) |
15. Bottom line
Digging deeper buys Iran time but not immunity. Above ≈150 m the civil-engineering challenges, construction signatures, and operational handicaps balloon, while U.S. penetrator technology—as the MOP’s 2025 upgrades show—keeps closing the gap. Structural strength is only one variable; power feeds, ventilation, and logistics remain soft targets. Absent dispersion or diplomatic off-ramps, Tehran risks a perpetual “cat-and-hammer” stalemate in which each expensive layer of depth merely triggers the next American counter-measure.