With its nuclear programme beset as never before by sanctions, sabotage and assassination, Iran must now make a new addition to its list of concerns: One of the biggest conventional bombs ever built.
Boeing's 30,000-pound Massive Ordnance Penetrator (MOP), an ultra-large bunker buster for use on underground targets, with Iran routinely mentioned as its most likely intended destination, is a key element in the implicit US threat to use force as a last resport against Iran's nuclear ambitions.
The behemoth, carrying more than 5,300 pounds of explosive, was delivered with minimal fanfare to Whiteman US Air Force Base, Missouri in September. It is designed for delivery by B-2 Stealth bombers.
Would that weapon, delivered in a gouging combination with other precision-guided munitions, pulverize enough rock to reach down and destroy the uranium enrichment chamber sunk deep in a mountain at Fordow, Iran's best sheltered nuclear site?
While the chances of such a strike succeeding are slim, they are not so slim as to enable Tehran to rule out the possibility of one being attempted, according to defence experts contacted by Reuters.
A "second best" result might be merely to block the plant's surface entrances, securing its temporary closure, some said.
One US official, speaking to Reuters on condition of anonymity, described an attack on the underground site, about 160 km (100 miles) south of Tehran near the Iranian holy city of Qom, as "hard but not impossible."
The United States is the only country with any chance of damaging the Fordow chamber using just conventional air power, most experts say.
Israel, the nation seen as most likely to attempt a raid, has great experience in long range bombing include its 1981 raid on the Osirak nuclear reactor in Iraq and a 2007 strike on a presumed nuclar facility in Syria.
But it lacks the air assets to reach Fordow's depths, and has no MOP-sized bunker buster. An Israeli raid would therefore likely require other elements such as sabotage or special forces.
The vulnerability of the chamber at Fordow, believed buried up to 80 metres (260 feet) deep on a former missile base controlled by the elite Revolutionary Guards Corps, came into sharper focus on Monday when the United Nations nuclear watchdog confirmed that Iran had started enriching uranium at the site.
The same day a State Department spokeswoman declared that if Iran was enriching uranium to 20 percent at Fordow this would be a "further escalation" of its pattern of violating its obligations under U.N. Security Council resolutions.
Western powers suspect the programme is aimed at developing the capacity to build a nuclear weapon. Iran says it is strictly for civilian uses.
Critics of Iran's nuclear programme tend to agree that military action against Iran's nuclear work would be their last and worst option. Not only would this risk civilian casualties, but Iran would seek to retaliate against Western targets in the region, raising the risk of a regional war and risking global economic turmoil.
Once it had recovered it would probably decide unequivocally to pursue a nuclear bomb.
Critics of the military option further point out that non-military pressure is increasing. Apart from tools of statecraft such as sanctions and diplomacy, covert means against Iran's nuclear work probably include sabotage, cyber attacks, measures to supply Iran with faulty parts and interception of nuclear supplies. It may also involve assassinations of nuclear experts such as Wednesday's killing of a scientist in Tehran.
A strike, furthermore, would only delay, not destroy, an Iranian nuclear programme whose known sites are widely dispersed and fortified against attack.
But Washington sees the plausibility of a US strike on Iran's main nuclear sites as a vital adjunct to the campaign of pressure. The narrow, technical question of whether such an attack is feasible is therefore central to strategy.
"You don't take any option off the table," US Defense Secretary Leon Pannetta said on CBS's Face the Nation television programme on Jan 8.
Asked on the same programme how hard it would be to "take out" Iran's nuclear capability, US chief of staff General Martin Demspey said: "Well, I'd rather not discuss the degree of difficulty and in any way encourage them to read anything into that. But I will say that our, my, responsibility is to encourage the right degree of planning, to understand the risks associated with any kind of military option, in some cases to position assets, to provide those options in a timely fashion. And all those activities are going on."
Asked if the United States could act against Iran's nuclear capability using conventional weapons, he replied: "Well, I certainly want them to believe that that's the case."
The credibility of that implicit threat got a freshening-up with the arrival of the big new bomb in the US arsenal.
Military satisfaction was evident.
As Air Force Brigadier General Scott Vander Hamm explained to Air Force Magazine, the MOP "is specifically designed to go after very dense targets-solid granite, 20,000 (pounds per square inch) concrete, and those hard and deeply buried complexes-where enemies are putting things that the President of the United States wants to hold at risk."
He said MOP "kind of bridges the gap" between conventional munitions and nuclear weapons in terms of the effects that it can create. Whereas in the past, "you'd have to break that nuclear threshold" to attack such HDBT (hard and deeply buried targets), "with the MOP, you don't have to," the magazine reported.
Four months on from the bomb's arrival in the US arsenal, the Fordow announcement has sharpened the Western strategic focus on US military capacity.
Experts differ on the extent of the challenge at Fordow, but all agree it presents greater complexity than Iran's other underground site at Natanz, 230 km (140 miles) south of Tehran where enrichment happens in a chamber estimated to be 20 metres underground, or less than a third of Fordow's presumed depth.
The other likely targets are Iran's uranium ore processing plant at Isfahan, some 400 km (250 miles) south of Tehran and plutonium producing research reactor under construction at Arak 190 km (120 miles) southwest of Tehran. They are both above ground and considered vulnerable to attack.
Austin Long, an assistant professor at the School of International and Public Affairs at Columbia University, told Reuters the arrival of the MOP "does not solve the Fordow problem but it does make it easier".
Many experts are sceptical.
Mark Fitzpatrick, an Iran expert at London's International Institute for Strategic Studies, said that Natanz was buried under several layers of dirt and concrete but it was "nevertheless possible to damage it with precision bombing with one sortie to create a crater and second sortie to burst through the bottom of the crater to the facility below."
But the chamber at Fordow might be "impenetrable", he said, due to its presumed depth.
His doubts were echoed by Robert Hewson, Editor of Jane's Air-Launched Weapons, to Reuters, who said it was likely that Fordow had been built to survive a sustained assault.
"We know for a fact - or as near a fact as possible - that you will not be able to stop this programme with air strikes. There continues to be a whole lot of hysterical posturing about this. In the meantime, it keeps backing the Iranians into a corner," he said.
"Given that it (Fordow) is a relatively recent development, it has probably been designed with a lot of attention to protecting it against conventional strikes. You don't necessarily have to obliterate it, mind. You could block the exits, block access to power, isolate it from life outside, and then you have effectively switched it off.
DESTRUCTION, OR MERELY A SETBACK?
"But for sure it will have been designed with all of that in mind, and the Iranians will have done the best job they can to make it survivable."
Sam Gardiner, a retired USAF colonel who runs wargames for various Washington agencies, told Reuters a major problem was simply a lack of confirmed information about the Fordow plant.
"With the Natanz facility, as it was being constructed, satellites gave us the information on where and how deep enrichment was to take place. Fordow on the other hand is an unknown. Where is the enrichment chamber? How deep? Which direction does the tunnel go?"
"For Israel, or even the United States, destruction would be very difficult. The entrance to the underground tunnel can be shut, but that would only be a temporary set back."
Diplomats point out that International Atomic Energy Agency inspectors visit Fordow and are familiar with its layout. While their work is confidential, it is widely believed that Western intelligence agencies have some knowledge of the site's interior.
John Cochrane, a defence specialist at the London-based Exclusive Analyst risk consultancy, said he believed the bunker-busting MOP might make a difference. But he suggested Fordow was at the very limit of the bomb's capacities, which he said could reach down to a maximum of 60 metres.
"Repeated strikes by Tomahawk cruise missiles and MOP might be effective in penetrating the site, if it is not as deep as 80m but, even then, we question whether an attack would have the same level of assurance in terms of damage as strikes on other 'softer' sites," he told Reuters.
"We question from what little we have seen of open source imagery whether it is as deep as 80 metres. If it is, we don't know for a fact but we think that is probably too deep for any form of air-delivered munitions, including MOP Cyber attack or physical assault by Special Forces may be the only attack options."
Cochrane noted that the supply of the MOP to Israel, even if the US were prepared to release it, would also require a suitable aircraft to deliver it and Israelis did not have one.
ATTACKING "THE HARD WAY"
In a 2010 study titled "Options in Dealing with Iran's Nuclear Program," analysts Abdullah Toukan and Anthony Cordesman of the US think tank the Center for Strategic and International Studies found that, if all peaceful options had been exhausted, the US was the only country that could launch a successful military strike.
Even that study predicated its finding on a strike merely blocking Fordow's two entrances, not destroying the underground chamber.
But in a Nov. 2011 article in Israel's Tablet magazine, Columbia Univeristy's Long concluded that Israel had the ability to attack the Fordow site using 75 bunker busters, each delivering a smaller explosive charge of about 1,000 pounds. However, he said it would require an unprecedented level of precision.
Long's scenario sees Israeli jets having "to do things the hard way", delivering 75 bunker busters on a single point to burrow through the rock.
There were two principal challenges, he said.
First, the weapons themselves, dropped from miles away and thousands of feet in the air, had to arrive at very close to the same angle to create a pathway each subsequent weapon could follow, he wrote. "Otherwise much of the penetrating power of the bombs will be wasted".
The second unknown was the "spoil problem", where the sides of the pathway, destroyed by previous explosions, clog the pathway for subsequent bombs.
Long subsequently told Reuters in emailed remarks the main feedback he had had from militry readers was that "the kind of operation I discuss is really, really hard to coordinate."
"I agree, though I don't think that makes it impossible, just very difficult, as I noted."