That has led many experts outside Iran to this conclusion: If the Iranians want to produce fissile material for weapons, they would be likely to do so at a hidden site. Natanz represents their first fuel cycle. This hidden site, or sites, would constitute the second.
The US intelligence community has predicted this. Two years ago, in their 2007 National Intelligence Estimate of Iran's nuclear intentions and capabilities, US analysts said: "We assess with moderate confidence that Iran probably would use covert facilities – rather than its declared nuclear sites – for the production of highly enriched uranium for a weapon."
So, is Qom this covert facility?
Iran says that it is not and that, like Natanz, it is for civilian uses.
If it were intended to be a secret site, then it is likely that somewhere is also a hidden facility for producing uranium hexafluoride, the centrifuges' feedstock. In fact, some proliferation experts suggest that Iran may have a network of such sites. Or, if it does not already have such a network, Tehran may now start building one, because Qom has been discovered.
That would make keeping tabs on Iran's program something like a high-stakes game of Whac-A-Mole. Miss, and perhaps Tehran gets a nuclear arsenal.
"They hid Qom, and our intelligence agencies found it in the nick of time," says David Albright, a former weapons inspector and president of the Institute for Science and International Security. "We can't count on that next time."