“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.

***

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.

***

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.