(TNND) — Air Force Gen. Dan Caine, chairman of the Joint Chiefs of Staff, told reporters Thursday morning that "Operation Midnight Hammer," the strikes against Iranian nuclear sites, was the culmination of 15 years of intense work by the American military.

And Caine said the bunker-buster bombs used in the attack were tailor-made to destroy Iran's deeply buried Fordo enrichment facility.

“In 2009, a Defense Threat Reduction Agency officer was brought into a vault at an undisclosed location and briefed on something going on in Iran,” said Caine, standing alongside Defense Secretary Pete Hegseth for a media briefing on last weekend’s U.S. strikes.

Caine said the Defense Threat Reduction Agency, or DTRA, houses the world's leading experts on deeply buried underground targets.

And the Joint Chiefs chairman offered extraordinary detail of how U.S. forces had spent years preparing for the possibility of dropping bombs on the Fordo site.

Caine and Hegseth said the massive American bombs hit their target and worked perfectly.

Caine said they decided to strike a pair of ventilation shafts as the primary point of entry to Fordo.

The general said the Iranians attempted to cover the shafts with concrete caps in the days before the U.S. strikes.

But Caine said that didn’t do much to slow the 30,000-pound GBU-57 bombs dropped by B-2 stealth bombers.

“The planners had to account for this. They accounted for everything. The cap was forcibly removed by the first weapon, and the main shaft was uncovered,” Caine said. “Weapons two, three, four, five were tasked to enter the main shaft, move down into the complex at greater than 1,000 feet per second, and explode in the mission space. Weapon No. 6 was designed ... as a flex weapon to allow us to cover if one of the preceding jets or one of the preceding weapons did not work.”

Caine showed reporters a video of the bombs' destructive power.

Caine said the GBU-57 bombs won’t leave an impact crater like normal surface bombs, because they’re designed to deeply bury and then function.

And some were designed with longer-delayed fuses to do maximum damage deep underground.

“In this case, the primary kill mechanism in the mission space was a mix of over-pressure and blast, ripping through the open tunnels and destroying critical hardware,” he said.

Caine said the work for this mission began with that highly classified intelligence briefing in 2009.

“For more than 15 years, this officer and his teammate lived and breathed this single target, Fordo, a critical element of Iran's covert nuclear weapons program,” Caine said.

The general said it was clear to them that Iran was up to something.

“You do not build a multi-layered underground bunker complex with centrifuges and other equipment in a mountain for any peaceful purpose,” he said.

Those DTRA officers watched and analyzed every aspect of Fordo, Caine said.

“(They) looked at the vent shaft, the exhaust shaft, the electrical systems, the environmental control systems, every nook, every crater, every piece of equipment going in, and every piece of equipment going out,” Caine said. “They literally dreamed about this target at night when they slept.”

The GBU-57 bombs were developed because those officers realized the U.S. didn’t have a weapon capable of destroying Fordo.

The U.S. military worked with the defense industry, tacticians and scientists to develop the bombs and test them over and over.

Caine said the weapons development resulted from supercomputer-intensive modeling and simulation and hundreds of real-world test shots.

When President Donald Trump made the call to bomb Iran’s nuclear sites, the air crews, tanker crews and weapons crews went to work.

“President Trump directed the most complex and secretive military operation in history, and it was a resounding success resulting in a ceasefire agreement and the end of the 12-day war,” Hegseth said at the news conference.

The defense secretary reaffirmed the administration’s belief that the targets were destroyed, and he admonished reports from a leaked “low confidence” and “preliminary” report that cast doubt on the effectiveness.

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Caine said the intelligence community will “grade” their work.

But he said all aspects of the attack went as planned.

“The trailing jets saw the first weapons function, and the pilots stated, quote, ‘This was the brightest explosion that I've ever seen. It literally looked like daylight,'" Caine said.

Caine expressed his pride in the bomber crews and other service members who carried out the mission, which launched from the B-2 base in central Missouri.

"One last story about people,” the general said. “When the crews went to work on Friday, they kissed their loved ones goodbye, not knowing when or if they'd be home. Late on Saturday night, their families became aware of what was happening. And on Sunday, when those jets returned from Whiteman (Air Force Base), their families were there, flags flying and tears flowing. I have chills literally talking about this.”

And Caine offered a warning to America’s foes as he wrapped up his comments.

“Our adversaries around the world should know that there are other DTRA team members out there studying targets for the same amount of time,” he said.