Biggest non-nuclear bomb ever used: Inside GBU-57 attack on Iran’s nuclear sites

Contents

1. What is the GBU-57 and how was it developed

2. What is known about the GBU-57 bomb

3. How is the MOP bomb delivered

4. Technical characteristics of the GBU-57

5. What was attacked with GBU-57 bombs

6. Was the attack on Iran's facilities with MOP bombs successful?

What is the GBU-57 and how was it developed?

The GBU (Guided Bomb Unit) is a series of American precision aerial bombs that use different guidance systems or combinations of them.

The GBU-57, also called the MOP (Massive Ordnance Penetrator), is a guided bomb designed to destroy targets deep underground. Boeing manufactures it.

Work on this 30,000-pound (13,600 kg) bunker-buster bomb began in 2002 but was paused due to technical problems. However, the Iraq war revealed weaknesses in American anti-bunker weapons — they lacked enough penetration and destructive power. This renewed interest in developing large anti-bunker bombs.

In April 2011, the U.S. Air Force ordered eight GBU-57 bombs and supporting gear for $28 million. By September, the military received 20 bombs for B-2 bombers. In 2012, Congress approved $81.6 million to further develop and improve the weapon.

In January 2015, the Pentagon conducted a training bombing using the GBU-57. They simulated an attack on a uranium enrichment factory near the city of Qom. This factory is about 80 meters underground, likely reinforced with concrete, according to the American publication The National Interest.

By 2018, the GBU-57 was upgraded to better attack hard-to-reach and deep targets. Tests of the improved version took place in Missouri. In a video by The Aviationist, you can see a B-2 Spirit bomber drop the bomb, which opens stabilizers during flight. Seconds after it hits, a column of fire rises.

What is known about the GBU-57 bomb

The MOP bomb weighs 13.6 tons. About one-fifth of its weight is explosive, and the rest (80%) is made of metal casing. The bomb uses a mix of explosives.

photo: militaeraktuell.at

This bomb is special not just because of its size and weight, but because it can dig deep into the ground. It can break through concrete and steel before it explodes, making it very powerful against underground targets. The GBU-57 can go 60 meters into the ground and 16 meters into concrete before it blows up. It can do this because it falls from a great height and has a strong design that doesn’t break on impact. If several bombs are dropped one after another, each explosion helps the next bomb go even deeper.

The bomb has a GPS navigation system to make sure it hits the right target. Some reports say it also has electronic tools to fight against Iran’s electronic defenses.

The United States has never given this bomb to any other country — not even close allies like Israel.

How the bomb is delivered

Only one type of aircraft can carry this bomb — the American B-2 Spirit, also called the "stealth bomber." It is one of the most advanced American aircraft. The manufacturer says the B-2 can carry up to 18 tons of weapons. But the U.S. Air Force said the B-2 has been tested carrying two GBU-57 bombs at once, which weigh more than 27 tons. These bombers can fly 11,000 km without stopping and up to 18,500 km if they refuel in the air.

The BBC said that if the MOP bomb is used against a country with strong air defense like Iran, the B-2 would probably fly with other planes. For example, F-22 stealth fighters might be used to weaken air defenses, and drones could check the damage and see if more attacks are needed.

Technical characteristics of the GBU-57

• Length - 6.2 m,
• Diameter - 0.8 m,
• Weight - 13,609 kg,
• Warhead - 2,700 kg,
• Penetration depth - up to 60 m.

What was attacked with GBU-57 bombs

photo: Getty Images

The main target of the bombs was the uranium enrichment plant in Fordow. Iran has hidden this plant deep inside a mountain, but no one knows exactly how deep. Defense Express called this plant "the heart of Iran’s nuclear program."

Fordow is Iran’s second nuclear enrichment site after the main one in Natanz. It was built between 2006 and 2009. The plant is inside a mountainside near the city of Qom, about 100 km from Tehran, the capital.

In March 2023, the International Atomic Energy Agency (IAEA) found uranium at Fordow enriched to 83.7%. This is almost enough to make nuclear weapons.

The website The War Zone said that only the U.S. Air Force has conventional weapons that can destroy such deep targets like Fordow. The GBU-57 bomb is specifically designed for a strike like this.

Was the attack on Iranian facilities with MOP bombs successful?

The Wall Street Journal quoted former Pentagon official Mick Mulroy, who specialized in the Middle East. Mulroy said that if the U.S. were to take part in hostilities and attack the plants in Fordow and Natanz, it would take about 12 bunker-busting bombs to destroy them. Professor Paul Rogers of the Department of Peace Studies at the University of Bradford in the UK, in a comment for the BBC, estimated the operational stock of GBU-57 bombs in the U.S. as limited — "no more than 10-20".

After the attack, Fox News said 6 bombs were used. It’s still hard to know for sure what damage was done. Official Washington said the Fordow plant was completely destroyed. But Iran said they were expecting an attack and had moved all important nuclear equipment away in advance.

It’s also unclear if all the Fordow facilities were really destroyed because information about the underground plant was confusing. At first, people said Fordow was about 80 meters under rock and soil. But in early June 2025, new numbers came out. Rafael Grossi, the head of the IAEA, told the Financial Times that the most important parts of Iran’s nuclear program are as deep as 800 meters underground. He said he had visited these places many times and was sure they could not be destroyed by one quick strike.

Defense Express commentators said that if the depth is really 0.8 km, then regular high-precision weapons can’t reach the targets.

After the attack, Iran said it will not stop its nuclear program. Donald Trump said that Tehran should now make peace, but added, “If the Iran problem is not solved, the United States is ready to strike again.”