WASHINGTON — The U.S. military just dropped "the mother of all bombs," the GBU-43/B, on a tunnel complex used by the branch of the Islamic State group in Afghanistan, marking the first time the munition has been used in combat.

The MOAB" which actually stands for Massive Ordnance Air Blast, is the largest non-nuclear bomb ever dropped in battle. But don’t be fooled. It’s not the biggest one in the arsenal.

See the MOAB airstrike footage here.

The MOAB is an air burst weapon intended for soft to medium surface targets over an extended area, an Air Force official told Defense News on background. While it doesn’t have the penetrative capability that some other munitions have, it is optimized for as targets in environments like caves, canyons or an extensive mine fields.

That could be one reason why it was used Thursday. During the mission, an MC-130 piloted by Air Force special operators dropped the 21,600 pound, GPS-guided ordnance in the Achin district of Nangarhar province, Afghanistan, a Central Command news release stated.

"As ISIS-K's losses have mounted, they are using IEDs, bunkers and tunnels to thicken their defense," Gen. John W. Nicholson, the top commander in Afghanistan, said according to the release. "This is the right munition to reduce these obstacles and maintain the momentum of our offensive against ISIS-K."

One advantage of using the MOAB, as opposed to other precision guided weapons like a Tomahawk or JDAM, is extremely high explosive content that creates a large blast area over the surface of the battlefield, said Steve Zaloga, a senior analyst with the Teal Group. The pressure from that explosion likely detonated many improvised explosive devices.

"So it may just be a quick way to clear a large surface area of the IED threat while at the same time doing a great deal of damage to any ISIS troops that are in the area," he said. "This is not necessarily the kind of weapon that you use to very deep or very heavily reinforced caves, but obviously a blast this size will collapse smaller more vulnerable caves."

Many details surrounding the GBU-43 remain shrouded in mystery, including its cost and the total number of units purchased by the Air Force. The service declined to comment on how many of these munitions are left after the strike on Wednesday.

More is known about its development. MOAB was created to put pressure on Saddam Hussein's regime. The bomb was rapidly manufactured by the Air Force Laboratory, going from an idea to its final test — conducted on March 11, 2003 — in roughly a year, according to an Air Force news release. The first munition was delivered on April 1 that year, but was never used in the Iraq War.

Because of the size of the ordnance, which measures 30 feet long, it cannot be carried by bombers.

So what’s bigger than the GBU-43? That would be the GBU-57A/B Massive Ordnance Penetrator, or MOP. Like the MOAB, the MOP is guided by GPS, but it is much more massive at 30,000 pounds, and as  "bunker buster" is capable of penetrating deep into the ground to take out fortified bunkers or other installations.

The MOP was developed by AFRL and produced by Boeing. The first explosive testing was conducted in 2007, but it has never been used in combat yet.

Valerie Insinna is Defense News' air warfare reporter. She previously worked the Navy/congressional beats for Defense Daily, which followed almost three years as a staff writer for National Defense Magazine. Prior to that, she worked as an editorial assistant for the Tokyo Shimbun’s Washington bureau.

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