President Barack Obama’s nuclear diplomacy with Iran may yet fail. On Tuesday, exactly one week before a June 30 deadline for an agreement, Iran’s Supreme Leader delivered his latest in a series of defiant statements, setting conditions for a deal—including immediate relief from sanctions, before Iran has taken steps to limit its nuclear program—that Obama will never accept. Secretary of State John Kerry warned last week that the U.S. is prepared to walk away from the talks. And even if a deal is reached, the story is not over. The Iranians may break or cheat on an agreement, and try build a nuclear weapon anyway.

That’s why, at least three times in the past year, a B-2 stealth bomber has taken off from an Air Force base in Missouri and headed west to the White Sands Missile Range in New Mexico. For these missions, the $2 billion plane was outfitted with one of the world’s largest bombs. It is a cylinder of special high-performance steel, 20 feet long and weighing 15 tons. When dropped from an altitude likely above 20,000 feet, the bomb would have approached supersonic speed before striking a mock target in the desert, smashing through rock and burrowing deep into the ground before its 6,000 pounds of high explosives detonated with devastating force.


“It boggles the mind,” says one former Pentagon official who has watched video of the tests.

Those flights were, in effect, trial runs for the attack on Iran that President Barack Obama, or his successor, may order if diplomacy can’t prevent Iran from trying to build a nuclear weapon.

Think of it as Plan B for Iran. The failure of diplomacy might lead the U.S. to turn to a weapon finally ready for real-world action after years of design and testing. The so-called “Massive Ordnance Penetrator,” or MOP, represents decades of military research, dramatically accelerated in recent years, focused on the problem of destroying targets buried deep underground.

That research once revolved around places like Russia, Iraq and North Korea. But in recent years, aided by a little-known military team of intelligence analysts, geologists and engineers, it has come to focus on Iran. More specifically, a uranium enrichment facility burrowed more than 250 feet into a mountain, about two hours’ drive south of Tehran.

Iran’s facility, known as Fordow, houses 3,000 centrifuges that can enrich uranium to a purity suitable for nuclear weapons. Fordow is not Iran’s only enrichment facility, or even its largest. But it is the best protected. And it would be all Iran needs to develop a nuclear weapon.

The mock desert target was almost certainly meant to simulate Fordow.

When Obama officials say that “all options are on the table” to stop Iran from getting a nuke, they are in effect speaking in code about the MOP. The MOP is what Secretary of State John Kerry was clearly referring to when he recently told Israeli TV that the U.S. has “designed and deployed a weapon that has the ability to deal with Iran's nuclear program.” When CNN recently put the question directly — can the MOP destroy Fordow? — to Secretary of Defense Ashton Carter, he was succinct: “Yes. That’s what it was designed to do.”

Crews load the Massive Ordnance Penetrator in a test in March 2007 at White Sands Missile Range in southern New Mexico. | AP Photo

The Pentagon is otherwise coy. “We maintain military options should diplomacy fail,” is all said Col. Ed Thomas, spokesman for Joint Chiefs of Staff chairman Gen. Martin Dempsey, would tell POLITICO. But former government officials and experts spoke in far more detail. They described a wonder of military hardware that combines high technology with brute physics. It’s a weapon they hope will help intimidate the Iranians into making—and keeping—a deal.

“The message the Pentagon wants to send,” says Loren Thompson, a military consultant and analyst with the non-profit Lexington Institute, “is that there is no safe haven.”

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Military planners have studied ways to blow up would-be safe havens since at least World War II, when Britain employed so-called “earthquake” bombs with names like Grand Slam and Tallboy against buried German targets. After a lull during the Cold War, when ICBMs and thermonuclear bombs were all the excavation considered necessary for military targeting, targeted bunker busting came back into favor again when the 1991 Gulf War revealed Saddam Hussein’s elaborate underground network—including a bunker whose German architect boasted it could survive a nearby nuclear blast. The subject seized the Pentagon’s attention anew after September 11, particularly after the early hunt for Osama bin Laden in Afghanistan showed that even B-52 carpet-bombing could only rattle al Qaeda’s mountain lairs. One 2001 report to Congress estimated that there were more than 10,000 buried or “hardened” targets worldwide requiring specialized bombs—many of them in North Korea and China—and that the number was growing.

In August of 2002, an Iranian political opposition group revealed the existence of an underground uranium enrichment facility at Natanz. News of Iran’s secret nuclear program startled Washington, though the site was too shallow to escape the reach of existing American munitions.

The 2009 discovery of the enrichment lab at Fordow was another matter. Fordow is a much smaller facility than Natanz, but it is buried deeper, better reinforced, and protected by mountain rock. In Washington, the Pentagon hit the gas pedal on its bunker buster program and its ability to detect and analyze underground sites.

A locus of this activity is a secretive arm of the Pentagon known as the Underground Facilities Analysis Center (UFAC). Established in 1997 and run by the Defense Intelligence Agency (DIA), the UFAC does not have a website; a DIA spokesman offered few details about it. But it is no backwater. According to one 2009 defense industry report—one of very few news articles to acknowledge the center’s existence— the UFAC’s staff swelled from 20 employees to 240 in its first decade. The number is probably now much higher.

“There’s been a realization over the past several years that our adversaries are looking at some of the best ways to defeat US capabilities, and one of them is to build things underground,” says Austin Long, a professor at Columbia University’s School of International and Public Affairs who has closely studied the issue. “So this has been a non-trivial part of the military and defense enterprise for years now.”

As its name indicates, UFAC isn’t just about hunting underground sites. It’s also about understanding their structure and layout—and figuring out how to destroy them. In addition to intelligence analysts, its staff includes scientists, geologists and people with “expertise in engineering and in all sorts of construction,” according to DIA spokesman James Kudla. Though Kudla would not elaborate, other sources—including a former UFAC employee and declassified documents available through George Washington University’s National Security Archive—offer more detail about its activities.

Like doctors relying on X-rays and MRIs, cave hunters must rely on high-tech equipment that can, in effect, see through solid objects—or sense things like seismic disturbances. That can involve exotic-sounding devices like geophones, laser vibrometers and drones equipped with gravimeters—devices that sniff for gravitational disturbances which suggest an underground cavity. (The U.S. has never explained the mission of an unarmed U.S. drone that crashed in Iran in 2011.)

They also rely on more traditional means, including satellite imagery from the National Geospatial-Intelligence Agency (NGA). That can reveal, say, a new transportation route that suspiciously dead-ends at a mountainside. Analysts also hunt for so-called “effluents,” like liquid waste. Exhaust vents aren’t just a tell, but can also be a vulnerability: Long calls this the “Death Star problem” facing any bunker-builder, a reference to the air shaft that allowed Luke Skywalker to destroy Darth Vader’s massive space station with a single well-placed shot.

Hunched over computers in a commercial building near the Pentagon, UFAC workers analyze suspect sites and determine what it would take to blow them up. That could involve analyzing the soil and rock under which it is buried, its depth, the thickness of its walls and the materials used to reinforce them. Their conclusions are uploaded into targeting databases, including one code-named GEMINI, that are used by military planners worldwide. In the event of an attack, the UFAC, relying heavily on NGA imagery, would also analyze the damage and determine whether follow-up strikes were necessary.

Of course, it doesn’t always take a huge bomb disable a facility: The former UFAC employee notes that the center’s analysis sometimes amounts to saying, “All we gotta do is go over to this power plant and flip a switch” to disable a bunker’s power supply.

Fordow is not such a simple case.

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If you drive two hours south of Tehran you will come to the ancient city of Qom, one of Shiite Islam’s holiest cities. Carry on for another half an hour and you will come to Fordow, where the road will reach a security perimeter around a mountain, with anti-aircraft guns scowling at the skies. Beyond the outer fence, the highway branches into four paths which, as seen by satellite image, disappear into black holes in the mountain side.

When the Iranians began constructing Fordow nearly a decade ago, the U.S. could have done little damage to it. At the time, America’s most effective bunker-buster was the GBU-28, a 5,000-pound weapon capable of penetrating roughly 20 feet of concrete or about 100 feet of earth. Fordow is burrowed some 250 feet into the mountain. (Natanz is larger, with about three times Fordow’s 3,000 centrifuges; but at 70 feet deep it is also much shallower and far easier to strike.)

Even before the mountain facility was discovered, military experts were fretting about America’s limitations. Some proposed to use small nuclear weapons to devastate underground targets. But a Bush administration push for nuclear bunker-busters met fierce resistance in Congress, which killed funding for the idea. In response, the Pentagon doubled down on its conventional bunker-buster program.

That program has come a long way in the past decade. The Boeing-designed and produced MOP weighs six times more than the GBU-28, and about 15 times more than that bomb’s predecessor. According to published reports, the MOP can burrow through 200 feet of earth and 60 feet of concrete before its blast destroys whatever it finds there.

A B-2 Spirit bomber refuels from a KC-135 Stratotanker here during a deployment to Andersen Air Force Base, Guam. | U.S. Air Force Photo

As recently as 2012, however, even the MOP lacked the clout to take out Fordow, officials concede. Since then it has undergone repeated upgrades to remedy glitches. The Air Force’s B-2 fleet was upgraded, at a cost of nearly $100 million, to carry the bomb—but two 2013 test flights were aborted after faulty wiring prevented the planes from dropping the MOP. In the past several months, even as Obama’s nuclear diplomacy with Iran advanced, the MOP underwent further upgrades and refinements. Its fuse has been upgraded to ensure it can withstand the bomb’s initial impact with the earth, and its satellite guidance systems refined for more precise targeting. According to unnamed officials cited by the Wall Street Journal in April, the MOP has also been outfitted with countermeasures against Iranian jammers that might try to throw off its GPS system. (A former military official who spoke to POLITICO confirms the upgrades.)

It’s not clear how many of the bombs the Pentagon may now have. According to a 2012 defense industry newsletter report, Boeing delivered 20 MOPs at a reported total cost of $314 million. But at least $82 million has been budgeted for subsequent upgrades; throw in the cost of modifying the B-2 bombers and the price tag could easily be above a half-billion dollars.

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Imagine that the nuclear talks do collapse. Iran’s Supreme Leader insists that outsiders will never be allowed onto Iranian military bases to conduct spot inspections. John Kerry throws up his hands and flies back to Washington. President Obama issues a grave statement expressing his hope that peace is still possible. Perhaps Iran then begins accelerating its uranium enrichment at Fordow and Natanz, and intelligence reports suggest that Tehran has decided to try and build a bomb faster than the world can mobilize to prevent it. Or perhaps Obama is succeeded in 2017 by a Republican hawk who decides it's time to end the uncertainty about Iran’s program once and for all.

“I don’t believe that Obama would use this weapon,” says Michael Makovsky, a former Bush Pentagon official now at the Jewish Institute for National Security Affairs. “But it might well be a good tool for the next president.” (Israel would like the MOP, and might be more inclined to use it; but Obama has refused to share the weapon.)

If the order came from the White House, it would most likely summon Whiteman Air Force Base to action. Crews there would load the internal weapons bays of several B-2 bombers with MOPs. The giant stealth planes would then depart for their nearly 7,000-mile flight to mountainous western Iran. By the time the planes actually took off, the mission would likely be old hat to the pilots: A massive flight simulator at Whiteman includes a full-size replica of a B-2 cockpit mounted on hydraulics to mimic flight motion. Its realistic wraparound cockpit computer screen can be preloaded with highly detailed graphics showing the topography and target areas the flight crew would see during the flight, allowing them to practice the bomb run—or even the entire flight—under different weather conditions or times of day.

Once over Fordow at an altitude of 20,000 feet or more, the bombers would release their massive payload. As the enormous bombs fell, they would accelerate to phenomenal speeds of perhaps 700 miles per hour or more. Guided by satellite positioning, flexible tailfins would steer the MOP to a very precise impact point likely identified by the UFAC. The bomb would strike the rock with the tip of its sharply pointed nose. Its supremely reinforced casing would protect the fuse and explosives inside from the initial impact. In effect, a 15-ton, 20-foot nail would pound into the earth at the speed of sound.

Violent as that impact may be, it would hardly be enough to get the job done. The goal is for the MOP to drill dozens or even hundreds of feet through rock before exploding. That is made possible by smart fuses, whose blasts are triggered not by impact but by conditions like time, depth, or the presence of a void indicating that the bomb has broken through an interior ceiling.

Fordow is buried deep enough that a single MOP probably would not penetrate to the centrifuge hall deep inside. That’s why several bombers would likely drop their ordnance in succession, gradually smashing a tunnel of devastation towards mountain’s soft interior. GPS precision would enable several MOPS to be landed on virtually the exact same spot in rapid succession: the most powerful jackhammer in history. “You create a hole and then you drop another one down the hole,” says Long. Ideally, one of the MOPs would break through to the centrifuge hall and completely destroy it. But even short of a bulls-eye, multiple concussions could damage the delicate centrifuge cascades, or even collapse the interior chamber. “Several hitting in the same spot could probably defeat the facility,” Long says.

Not everyone is certain about that. “Can you take out Fordow with this ordnance? I doubt it,” says Anthony Cordesman, a military analyst at the Center for Strategic and International Studies. Cordesman notes that it’s hard to know how vulnerable an underground site may be, and that while some turn out to be flimsier than expected, the opposite is often true. Cordesman does believe, however, strikes targeted against its entrances could disable the facility for a substantial time. “Can you shut it down?” Cordesman asks. “Yeah.”

That’s a temporary solution. Entrances can be cleared and rebuilt. Replacing centrifuges and their nuclear material takes longer. Any president who risks an attack on Iran may insist on inflicting more damage than a few bulldozers can undo.

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Even if the MOP never touches Iranian soil, it will have played a vital function. Iran might not have come to the bargaining table without a credible threat to destroy its program. “Having the option and getting word of it out there reminds the Iranians that their program is at risk,” says the former Pentagon official. “That gives us leverage over them.”

“Even in the aftermath of a deal,” adds the official, “it creates a strong deterrent so the Iranians abide by the deal,” the official says.

It may be that Iran is too intimidated by the MOP to crank up enrichment at Fordow in the sight of international inspectors. But after any deal, the UFAC will scour Iran for any hint of the next Fordow.

That fear was articulated by a 2007 U.S. National Intelligence Estimate, which deemed that “Iran probably would use covert facilities—rather than its declared nuclear sites—for the production of highly enriched uranium for a weapon.”

In mid-May, a senior Iranian politician named Mohammad Javad Larijani said that, as long as the U.S. continues to speak of possible military action against Iran’s nuclear sites, his country should make them harder for America to destroy. Iran should build five new underground facilities, he said.

“Our facilities will not only remain underground,” Larijani said, “but will go even deeper in the ground.”

Perhaps those facilities, if built, would reach even beyond the MOP. At which point the UFAC analysts and the Pentagon will go back to the drawing board. And they will build an even bigger bomb to take them out.