The possibility of an Iran attack highlights the latest arms race: The United States trying to build new bunker-busting weapons while Iran buries its nuclear labs deep underground to try to avoid possible U.S. (or Israeli) bombs.

The United States and Iran have engaged in a war of words over their military capabilities in the last few weeks. But if an actual war breaks out, it will not be a war of U.S. bombs versus Iranian bombs, but of U.S. bombs versus Iran´s bunkers.

Iran´s network of nuclear facilities, some of which are underground, would be the primary target of an Israeli or U.S. attack intended to destroy Iran´s suspected clandestine weapons program. As the rhetoric has heated up, the United States has been talking up its military capabilities. Whether American or Israeli bombs indeed could penetrate or destroy Iranian bunkers, though, is still unclear.

The bomb getting the most press lately is the Pentagon´s newest megaweapon: the GBU-57 Massive Ordnance Penetrator (MOP), a 30,000- pound bomb designed to penetrate through dozens of feet of concrete. Speaking last week at a conference, Lt. Gen. Herbert Carlisle, the Air Force deputy chief of staff for operations, called it a great weapon and noted that it was "part of our arsenal."

But the MOP, which B-2 bombers will carry, is still in testing, though it is supposed to be ready for use sometime this year. Until the MOP is ready to go, the biggest bunker-busters in the U.S. arsenal are much smaller. That includes the 5000-pound GBU-28, a laser-guided bunker-buster (which Israel wants to acquire) that the U.S. developed two decades ago to obliterate bunkers in Iraq during the first Gulf War.

Meanwhile, Iran´s Press TV has cited public reports that Iranian industry is working on "ultra-high-performance concrete" that "may render Iranian nuclear sites impervious to U.S. bunker-buster bombs." Indeed, Iranian scientists are good at cooking up tough concrete, in part because so many earthquakes strike the country. But there´s no evidence that, even if Iran had such a super-concrete, it has been used to protect any of the nation´s nuclear operations.

Even so, the Pentagon is worried about what it calls Hard and Deeply Buried Targets, and with good reason. Iran´s Fordow facility, for example, is believed to be more than 265 feet underground and is part of a concerted strategy to distribute the country´s nuclear capabilities and make them more difficult to destroy. According to The Economist, Defense Secretary Leon Panetta has said that MOP would need an upgrade to destroy the deepest bunkers in Iran.

So besides the MOP, the Pentagon is also working on other weapons and technologies to break through underground bunkers, and a cottage industry has sprung up to build these new weapons. Last year, for example, the defense company ATK won a military contract for a technology called the Hard Target Void Sensing Fuze, which can be used to program a warhead to go off at a specific time. The fuze can be time-delayed to explode only after penetrating through many feet of concrete, and it will also include a "void sensing function," which would allow it to know when it has gone through the ground and penetrated into a targeted bunker so that it doesn´t detonate too early. The Air Force Research Laboratory is also considering more-far- out bunker-busting technologies that may come around in the next few decades, such as directed-energy weapons.

Finally, the standoff in Iran creates another problem for any attacking country to consider, beyond the bunkers or their depths underground. Some of Iran´s nuclear facilities are located near populated areas, making airstrikes politically difficult and potentially dangerous to civilians because an airstrike could introduce radioactive material into the atmosphere.

"I do think that if you popped a ton of HE [high explosive] inside a tunnel at Qom or elsewhere there would be a plume of UF6 [uranium hexafluoride] that could contaminate the surrounding area," says Peter Zimmerman, a nuclear physicist and former chief scientist of the Senate Foreign Relations Committee. "But the question is not whether you could detect contamination, but would it be high enough to be significant?"

In places like the Fordow nuclear facility, outside the city of Qom, any contamination would likely be far away from populated areas, Zimmerman says. But some facilities, such as the Bushehr nuclear power plant or the Isfahan uranium-conversion facility, are in or near cities. "The bombing of the Isfahan uranium-conversion facility would undoubtedly send radioactive plumes over the historic city of Isfahan, a mere 9 miles downwind," says Joseph Cirincione, president of the Ploughshares Fund, a nonprofit arms-control association.

Bombing those facilities would have huge consequences for the surrounding area, Cirincione argues. "We have never experienced such bombings in history," he says. "Up until now, they have been considered beyond the pale."