The U.S. military operation targeting Iran‘s Fordow facility was 15 years in the making, according to Gen. Dan Caine, the chairman of the Joint Chiefs of Staff.

The Defense Threat Reduction Agency, known as DTRA, has been tracking the construction and development of the facility in the mountainous terrain since 2009. An officer Caine did not name was tasked with understanding and studying every aspect of the facility, and was later joined by a partner.

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“He was tasked to study this facility, work with the intelligence community to understand it, and he was soon joined by an additional teammate. For more than 15 years, this officer and his teammate lived and breathed this single target, Fordow, a critical element of Iran’s covert nuclear weapon program. He studied the geology. He watched the Iranians dig it out. He watched the construction, the weather, the discard material, the geology, the construction material, where the materials came from,” Caine said. “He looked at the vent shaft. He brought up the electrical systems, the environmental control systems, every nook, every crater, every piece of equipment going in and every piece of equipment going out.”

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“They literally dreamed about this target at night, when they slept. They thought about it, driving back and forth to work, and they knew from the very first days what this was for,” he added. “You do not build a multilayered underground bunker complex with centrifuges and other equipment in a mountain for any peaceful purpose.”

The officers realized the U.S. did not have a weapon capable of destroying such an underground facility, prompting the development of the GBU-57 Massive Ordnance Penetrator bomb, which is specifically designed to target underground structures. 

“In the beginning of its development, we had so many PhDs working on the MOP program, doing modeling and simulation that we were quietly, and in a secret way, the biggest users of supercomputer hours within the United States of America,” the chairman added.

The United States had never dropped the 30,000-pound bomb aside from during training and development prior to the strikes in Iran last weekend, when all of the officers’ planning was actualized.

Ahead of the U.S. operation, Iran tried to cover three ventilation airshafts at Fordow with concrete to prevent an attack. The first MOP bomb “forcibly removed” it and with it uncovered bombs two through five were able to enter the main shaft and move down into the complex, Caine explained. They dropped a couple of other bombs, partially in case one of the first ones did not go as planned.

“Each weapon had a unique desired impact angle, arrival, final heading, and a fuse setting,” Caine said. “The fuse is effectively what tells the bomb when to function. A longer delay in a fuse, the deeper the weapon will penetrate.”

“Unlike a normal surface bomb, you won’t see an impact crater, because they’re designed to deeply bury and then function,” he added.

The Joint Chiefs played a video during the briefing of a recent test use of an MOP bomb.

The U.S. military also bombed the Isfahan and Natanz Iranian nuclear facilities, but Caine and Defense Secretary Pete Hegseth did not speak as much about the damage assessments there.

There are still questions about the level of damage the strikes caused, and whether the Iranians were able to move any uranium or equipment out of those facilities before the U.S. strikes, though Hegseth spoke up about the damage caused and downplayed the possibility Iran moved any of its enriched uranium out preemptively.

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An initial assessment from the Defense Intelligence Agency found with low confidence that the strikes had not been as impactful as Hegseth and President Donald Trump had claimed, though it noted that the opinion would be refined as more intelligence became available.

Hegseth, Trump, and several other administration officials rebuked the media for reporting on the leaked assessment, now the subject of a federal investigation.