The US and Israeli forces struck an ammunition depot in Iran’s Isfahan early Tuesday (March 31).

US officials, cited by The Wall Street Journal, said the strike involved 2,000-pound (about 907-kg) bunker-buster bombs targeting a military-linked site in Isfahan, a central Iranian city hosting key defence facilities, including the Badr airbase. It stated that a large number of penetrator munitions were used in the operation.

Reports said the initial strike triggered multiple secondary explosions across the area. Visuals released over the internet also showed the explosions.

The term “bunker buster” typically describes bombs designed to penetrate deep below the surface — layers of rock, earth, or concrete — before exploding. This hard-target munition is understood to have been developed during the Gulf War.

Who makes it, and what is its impact?

Developed by the US Defence Major Boeing, the US GBU-57 A/B Massive Ordnance Penetrator is the most powerful in this category and is a GPS-guided 30,000-pound (13,600-kilogram) bomb, which can penetrate about 200 feet (60 meters) of solid concrete.

However, for the attack on Isfahan, a smaller variant of the bomb (2000 pounds) was reportedly used, even as the purpose was the same — to hit hardened and deeply buried underground and reinforced structures.

In all likelihood, the GBU-27 variant was likely used in the attack, as it weighs about 2,000 pounds (900 kg). This variant is also less expensive and more numbers can be carried in each mission.

The GBU-57 A/B is understood to comprise a void-sensing fuze to detect the time the weapon has reached the target, such as an underground depot or bunker, following which the bomb detonates.

The United States had used the GBU-57 A/B in strikes on Iranian nuclear facilities during fighting between Israel and Iran in 2025. Fourteen of these, launched by B-2 Spirit stealth bombers, hit the Fordow and Natanz facilities to target subterranean nuclear infrastructure.

This bomb can only be dropped from a B-2 stealth bomber because of its sheer size. Only the US possesses this aircraft.