Today the world knows all about Iran's previously secret uranium enrichment facility near Qom. But the plant's disclosure raises another question: Are there more covert Iranian nuclear sites the West has yet to discover?
Perhaps not. But some experts believe it is possible that Tehran has built a shadow nuclear infrastructure in an attempt to produce fissile material – and perhaps weapons technology – without outside interference.
The existence of the Qom facility alone raises some tantalizing questions. If it is an enrichment facility, it would need raw material to work with, in the form of uranium hexafluoride. That's a compound produced from uranium ore in a conversion plant.
"The bottom line is, there is a natural and reasonable suspicion that there is a clandestine conversion plant as well," says James Acton, an associate in the nonproliferation program at the Carnegie Endowment for International Peace .
The Qom site may soon have its first international inspection. On Thursday, European Union foreign policy chief Javier Solana said that Iran plans to cooperate with the International Atomic Energy Agency (IAEA) on the matter, and that a visit could take place in a "couple of weeks".
The IAEA itself also announced on Thursday that its chief will visit Iran to discuss nuclear issues. An IAEA statement said that director Mohamed ElBaradei, has been invited by Iranian authorities, and will "travel there soon to discuss a number of matters".
From the standpoint of uranium enrichment capability, the Qom plant appears to be relatively small. It is capable of holding only about 3,000 spinning centrifuges.
If there are more secret enrichment plants, Iran could move more quickly towards a nuclear arsenal, Mr. Milhollin noted in a Wednesday New York Times opinion piece written with Valerie Lincy of Iranwatch.org . And if the Qom facility is indeed part of a secret plan to make nuclear weapons, then somewhere in Iran there may be laboratories and workshops to produce "the first sets, high-explosive lenses and other necessary parts" for a nuclear weapon, wrote Milhollin.
As to feedstock for the Qom plant, it might come from Iran's existing uranium conversion plant at Esfahan. But this facility has long been open and under the watchful eye of IAEA inspectors.
The amount of uranium hexafluoride necessary for Qom's centrifuges is large enough that such a diversion should be easy for the IAEA to spot, says Mr. Acton of the Carnegie Endowment.
Given that Tehran has built secret nuclear sites in the past, it would be consistent for Iran to have constructed a secret conversion plant in parallel with Qom, says Acton. "The bottom line is, we don't know."
Iran nuclear talks in Geneva: What is Tehran's strategy?
Follow us on Twitter.