The long suspected nuclear site is located on the Parchin military base, which is why the Iranians claim that the IAEA has no business inspecting it.
Now it appears that the Iranians are in the process of cleaning it up in naticipation that inspectors will be taken in for a visit.
A U.S. think-tank published satellite imagery on Friday which it said showed "pink colored material" covering a building at a sensitive military site in Iran which U.N. nuclear inspectors want to visit.
The Institute for Science and International Security (ISIS) said the tarp - at the Parchin military complex - might be an attempt to conceal alleged clean-up work there at a time when Iran is under growing international pressure to open up its disputed nuclear program to scrutiny.
The images were consistent with a Reuters story on Thursday, which cited diplomatic sources as saying a brightly-colored tent-like structure now covered the building.
The U.N. International Atomic Energy Agency (IAEA) believes that explosives tests relevant to the development of nuclear weapons were conducted in a steel chamber in the building, possibly a decade ago, and wants to go there to search for evidence.
The IAEA pressed Iran again in talks on Friday to address concerns about suspected nuclear weapons research in the Islamic state, repeating requests for access to Parchin, but their meeting ended without an agreement.
In recent months, Western diplomats have cited satellite pictures showing extensive sanitization efforts at the site in what they describe as an apparent attempt to remove any evidence of illicit nuclear activity.
Iran, which denies Western allegations that it is seeking a nuclear weapon capability, says Parchin is a conventional military facility and has dismissed allegations about it as "ridiculous."
ISIS, which tracks Iran's nuclear program closely, said commercial satellite imagery it had obtained from August 15 "clearly shows the suspected high explosive test building covered in a pink-colored material."
The worry is that Iran moved the equipment to some other building in the large, sprawling complex and it will be difficult to ferret out the new location. Or they could have moved the experiments off site entirely.
Either way, it appears Iran is doing everything it can to keep from being discovererd with smoking gun evidence that it was working on nuclear weapons.