The Islamic State fortress that was hit with a massive U.S. bomb Thursday was reportedly nearly impenetrable, until the “mother of all bombs” flattened it.

The mountain fortress, located in the Achin district of Afghanistan, was reportedly heavily fortified and extremely difficult for ground forces to reach, according to Afghan Gen. Daulat Waziri, spokesman for the Afghanistan Ministry of Defense.

“It was a strong position and four times we had operations (attacking the site) and it was not possible to advance,” Waziri told the Associated Press Friday.

Waziri noted that the fortress contained tunnels more than 12o feet deep into the ground. The road leading up to the position was also “full of mines.” The fortification’s defenses were such that the use of the MOAB was necessary, according to the general.

The Pentagon released a video of the strike Friday.

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Achin district is located in Afghanistan’s Nangarhar province, a mountainous region that shares a border with Pakistan. The province is the center of ISIS activity in Afghanistan.

The U.S. believes the group, known as Islamic State Khorasan (IS-K), has only 600 to 800 fighters. IS-K is significantly smaller than the Taliban and al-Qaida, but it has successfully conducted terrorist attacks in Afghanistan. Approximately 36 ISIS fighters were reportedly killed in the attack.

The GBU-43B is approximately 30 feet long and can unleash 11 tons of explosive ordnance on a target. Technically, the term “MOAB” comes from an official designation: Massive Ordnance Air Blast, the “mother of all bombs” is simply a nickname.

The bomb achieves maximum destruction by detonating six feet off the ground, flattening anything in its wake. It was first tested in 2003, but the Nangarhar strike represented the first time the huge weapon has been used in combat.

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