A US Air Force B-2 Spirit in the sky over northwest Missouri in August 2018.

A US Air Force B-2 Spirit in the sky over northwest Missouri in August 2018. US Air National Guard photo by Senior Master Sgt. Vincent De Groot

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The US’s “massive precision strikes” on three Iranian nuclear facilities unleashed the GBU-57 Massive Ordnance Penetrator bunker-buster bomb for the first time in combat.

In a press briefing Sunday morning, Gen. Dan Caine, the Chairman of the Joint Chiefs of Staff, said US forces fired around 75 precision-guided weapons in total during the operation, which targeted facilities at Fordow, Natanz, and Esfahan. The weapons included 14 GBU-57s.

The GBU-57, designed to penetrate hardened bunkers, particularly those located deep underground, is among the heaviest and most powerful non-nuclear bombs in the US arsenal.

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Here’s what we know about them.

The US Air Force says the bomb is designed to target “weapons of mass destruction located in well-protected facilities.” Mehmet Yaren Bozgun/Anadolu via Getty Images

The GBU-57 weighs roughly 30,000 pounds and can only be carried by the B-2 Spirit bomber. It is approximately 20.5 feet long and has a diameter of 31.5 inches.

The US Air Force says the bomb is designed to target “weapons of mass destruction located in well-protected facilities” and can penetrate up to 200 feet — or roughly two-thirds the height of the Statue of Liberty — below ground before exploding. The depth it can travel depends on the strength of the material it is penetrating.

A mock-up of the Massive Ordnance Penetrator sits in the bomb bay of the B-2 weapons load trainer at Whitman Air Force Base, Mo. U.S Air Force.

Iran’s Fordow facility is thought to be a key site for Tehran’s nuclear enrichment activities, but its location — buried deep under a mountain — means Israel likely lacks the firepower to take it out on its own, according to Heather Williams, a senior fellow in the Defense and Security Department at the Center for Strategic and International Studies.

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Multiple hits by a GBU-57, however, could destroy it, she said.

President Donald Trump, who said late Saturday that a “full payload” of bombs had been used to strike Fordow, praised US forces for what he called a “very successful attack.”

The US strikes involved 14 GBU-57 bunker-buster bombs. Murat Usubali/Anadolu via Getty Images

Trump called for peace in the aftermath of the US strikes on the nuclear sites.

The attacks are a significant escalation in the ongoing conflict. Earlier this month, Israel launched an intense air campaign against Iran aimed at degrading its nuclear capabilities. Tehran, which has said its nuclear program is for civilian purposes, has retaliated against Israel with heavy missile and drone strikes.

Trump said earlier this week that he was considering US involvement and indicated he’d take two weeks to mull over decisions to bomb Iran’s nuclear facilities. He appears to have determined that less time was necessary.

Iran had threatened to retaliate against the US and its forces in the Middle East if it got involved in the Israel-Iran conflict. The US armed forces have a substantial force presence in the volatile region that the Department of Defense has said is intended to protect and defend American interests.