Will Iran ever let international nuclear inspectors visit the Parchin military complex near Tehran?
The question dominates every talk between the two sides, and this week's board of governors' meeting of the International Atomic Energy Agency (IAEA) is no exception.
Director General Yukiya Amano opened the weeklong session by calling again on Iran -- an IAEA member -- "to provide access" to Parchin. It is a call he has made for over a year that has gone unheeded.
Nuclear inspectors said in November 2011 they believe Iran conducted experiments at Parchin more than a decade ago "relevant to the development of an explosive nuclear device." They suspect Iran built a container at Parchin in 2000 designed to contain the detonation of up to 70 kilograms of high explosives used to develop models for triggered nuclear reactions.
But since then, the agency has been steadily refused permission to visit the site, even though it visited the Parchin base -- but not the controversial building -- once time in 2005.
Iran's refusal only increases Western suspicions that Iran has something to hide, despite Tehran's insistence that Parchin is a military facility used solely for conventional-weapons testing.
Inspectors are far from losing interest because there is other evidence that makes the site controversial.
"It also became known that there was this Russian scientist from one of the Russian nuclear labs who is an expert on explosives and was working at that site," Elena Sokova of the Vienna Center for Disarmament and Non-Proliferation says.
But what heightens inspectors' suspicions most is Tehran's extensive cleanup work at the disputed building site over the past year. That has included dismantling the original building, removing soil and, according to latest reports, beginning construction on a replacement building.
All that, Sokova says, seems intended to frustrate any future attempts by inspectors to reconstruct the history of the site if they ever do get access.
Not all analysts suspect that Parchin was indeed part of a nuclear program that the United States intelligence community believes Iran had until 2003 and that some countries, including Israel, believe may continue today.
One retired IAEA nuclear inspector, Robert Kelly, a research fellow at the Stockholm International Peace Research Institute (SIPRI) in Sweden, has proposed that the earth berm around the original building is too small to suggest high explosives were used there.
But whether or not the site was actually used for nuclear-related testing, the fact Iran refuses access to it -- and tampers with it -- makes Parchin something bigger than just a key target for investigations. It has become the symbol of the powerlessness of the IAEA to make progress toward solving the Iran nuclear crisis.
Shannon Kile, a nuclear expert at SIPRI, calls that ironic. Originally, most observers thought Parchin would be one of the easier sites for the two sides to agree upon visiting.
"This was a case where everyone thought that going to Parchin was something that the IAEA and the Iranians would be able to agree on [because] the IAEA had visited Parchin in 2005," Kile says. "No one thought this was going to be a serious hang-up. And because of the broader deterioration of relations between Iran and the IAEA, this has unfortunately become a very public and prominent symbol of the deadlock or the impasse between the agency and the Iranian government."
As Parchin becomes a line in the sand for both sides, few people believe that it, or any other physical site in Iran, will finally provide the conclusive evidence that inspectors need to finally settle the question of whether Iran has pursued nuclear weapons.
Kile says that kind of evidence can only come from people and documentation -- things that can't be covered up or easily altered.
"If there is any evidence, it is going to come from interviews, from looking at documentation of past activities," Kile says. "I suspect they are not going to find, for example, some secret nuclear-weapon, warhead-production facility. I think that is not even what they are really looking for."
So far, the dispute over Parchin has effectively stopped any chance of investigators getting Iran to voluntarily give them access to the scientists and documents they seek. And to judge by the continuing stalemate in Vienna, it could do so indefinitely.