What Is Massive Ordnance Penetrator, US Bunker-Busting Bomb?

Washington: The United States entered the escalating conflict between Israel and Iran as it used its B-2 stealth bombers in its strikes on Iranian nuclear facilities.

• Trump says US attack 'obliterated' Iran nuclear sites
• Iranian Foreign Minister says US attack 'unforgivable', no room for diplomacy 'right now'
• Qatar regrets deterioration of situation following attack on Iranian nuclear facilities
• Kuwait, Saudi Arabia confirm stable radiation levels

A small number of Air Force strategic bombers took up the task to deliver 30,000-pound (13608-kg) precision-guided bombs designed to destroy subterranean targets.

This bomb, the GBU-57, is better known as a "bunker buster” or the Massive Ordnance Penetrator.

Why is the MOP the right tool for the job?

It's in the name.

The MOP is designed for deeply buried and fortified facilities, such as bunkers and tunnels.

Its design, sheer weight and steel alloy construction allow it to burrow underground and then explode, according to the US Air Force.

Though the heaviest conventional weapon in the US arsenal, it is not designed to saturate explosives over a wide area.

Commanders rely on its GPS-guided precision to hit specific, well-defended targets to destroy what ordinary bombs cannot reach.

There are no public reports of the MOP being used in combat, experts have said.

Defense officials have said the MOP is capable of penetrating up to 200 feet (60 metres).

Massive Ordinance Penetrator MOP GBU 57 Bunker Buster Bomb. Photo shared by visegrad24 on X.

But it is probably more capable now after further development over the past two decades, said Trevor Ball, a former Army explosive ordnance disposal technician.

While the Israelis have relied on US munitions for their devastating air war in Gaza, Lebanon and Iran, their fighter jets cannot carry MOPs.

The US B-2 Spirit stealth bomber is the only Air Force aircraft that can deploy the MOP, the service has said.

There are 19 operational B-2s, according to the Air Force.

Traveling at subsonic speeds, but capable of midair refueling, the B-2 can fly an extraordinary distance.

During the Kosovo war in the late 1990s, B-2 pilots flew round trips from their home station at Whiteman Air Force Base in Missouri to strike targets.

In 2017, a pair of B-2s flew 34 hours to hit Islamic State camps in Libya.

Recent upgrades to the MOP include resolving an undescribed "integration issue” with the B-2, the Air Force said last year.

The service also said it is testing technology that can help destroy targets where intelligence about substructures may be limited.

A smart fuse on the MOP can detect voids on its path downward - such as rooms and floors - and explode at an optimal point, Ball said.

That would be an important capability if commanders decided they needed to strike the same deep target multiple times.

It is unclear whether that technology has been put into operational use.

The New York Times reported the B-2s used in the attack“flew non-stop for about 37 hours” from a base in Missouri and refuelled several times in mid-air.

Underground Iranian sites that were targeted

Iran's most deeply buried nuclear enrichment site is Fordow, in the desert southwest of Tehran.

This handout satellite image released by Planet Labs on June 13, 2025, shows the Fordow Fuel Enrichment Plant, northeast of Qom, Iran on May 19, 2025. Photo by Handout / © 2025 PLANET LABS PBC / AFP.

The facility is fully underground, carved into a mountainside.

UN inspectors who viewed the site noted tunnels with thick walls and blastproof doors, with some bunkers protected by up to 300 feet of rock, The Post reported in 2012.

Experts warn that even destroying the uranium-enriching centrifuges deep underground at Fordow would not necessarily mean the end of Iran's nuclear program.

There may be enrichment sites or caches of nuclear fuel that UN inspectors are unaware of, said Richard Nephew, a lead US negotiator with Iran under the Obama administration and now a fellow at the Washington Institute for Near East Policy.

"Even if Fordow was evaporated tomorrow, we would still have massive concerns,” Nephew said.

Fox News reported that "six huge bunker buster bombs" were used in the hit on the Fordow facility.

It further reported that Iran's Natanz and Isfahan nuclear sites“were wiped out with 30 Tomahawk missiles launched by American submarines some 400 miles away”.

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