The mysterious explosion in July, which destroyed a meeting hall for centrifuges in the most important Iranian nuclear fuel enrichment facility in Natanz, was classified by the Iranian authorities as hostile sabotage and triggered a defiant reaction: the destroyed building would be in the “heart of the mountains” to be rebuilt. Said the head of the Iranian Atomic Energy Agency.

The progress made on this promise, which could protect the facility from an air strike or other threats, was unclear to outside observers. But new satellite imagery is now shedding light on Iranian plans.

The New York Times Visual Investigations team followed the construction work on the site using the new images. For the first time, new tunnel entrances for underground mining are visible under a ridge in the foothills south of the Natanz facility, about 140 miles south of Tehran.

The Times worked with Jeffrey Lewis, an arms control expert at the Middlebury Institute of International Studies in Monterey, California, to interpret the new image.

“The new facility is likely a far safer place to assemble the centrifuge. It is far from a road and the ridge presents significant congestion that would protect the facility from air raids,” said Lewis in written comments.

The July explosion was not the only recent incident that appears to have exposed major security loopholes in Iran’s nuclear program, to which the country restricts only peaceful purposes. In late November, a brazen daylight attack killed leading Iranian nuclear scientist Mohsen Fakhrizadeh.

Iran has blamed Israel and the United States for the Natanz explosion and the Fakhrizadeh assassination, both of which were viewed as serious setbacks for the Iranian nuclear program.

Mr. Lewis described the evidence that underground construction was being carried out at the Natanz site.

“There appear to be two tunnel entrances on either side of a large ridge, with heaps of spoil from excavations nearby. The space between the two entrances is large enough to accommodate a facility roughly the size of the centrifuge assembly building that was destroyed this summer, and Iran said it will be rebuilt in the mountains. “

Changes can be tracked by looking at satellite imagery taken over several months. Even something as simple and inconspicuous as a pile of dirt is a clue.

“The main clue is the trash heap from the excavation that wasn’t there in July,” said Lewis. “Iran has also re-evaluated two streets on either side of the ridge that lead to what appears to be tunnel entrances.”

Allison Puccioni, an image analyst with the Center for International Security and Cooperation at Stanford University, pointed to other telltale signs of excavations near the heap of rubble. Commenting on the Times, Ms. Puccioni said the images between the heap of rubble and the excavation site showed “traces of excavated earth that were lighter than the existing busy road”.

The numerous activities at Natanz that have been tracked by satellites in recent months include building new roads and additional excavations that began after the explosion. Some analysts say additional tunnels are being built, suggesting that an even larger underground complex is being worked on.

The destroyed building was built in 2012 and was used to assemble centrifuges, machines for enriching uranium, which are needed for peaceful purposes – and, if they are enriched to a higher level, for bombs. The 2015 nuclear deal between Iran and the world powers stopped high-level enrichment, but Iran began accumulating enriched uranium again after President Trump left the deal two years ago.

The ability to see via satellite what Iran has been doing since the Natanz explosion partly reflects the quantum leap in this visual technology over the past two decades.

In 2002, analysts revealed the construction of the then secret Natanz enrichment facility using commercial high-resolution satellite imagery. Such sharp images were only available in 2000. At the time, the analysis required finding Natanz on Persian maps in the Library of Congress, faxing an order form, and waiting weeks to receive satellite images on a CD-ROM. Eighteen years later, analysts and journalists can quickly track even the smallest changes at a location like Natanz on their laptops.

These monitoring functions can create their own challenges. Frequently collected images do not show any completed construction work, but are in progress. The first interpretation of the latest changes in Natanz from October focused on a former shooting range south of the main facility as a possible location for the new assembly hall for the underground centrifuge. However, it turned out that these changes were a construction support system for road works and tunnel excavations.

In response to the Fakhrizadeh assassination, Iran enacted law last week to immediately increase uranium enrichment and ban international inspectors until February if US sanctions are not lifted. The law also requires the installation of advanced centrifuges in its nuclear facilities, including Natanz.

The United States may have been aware of recent attacks on Iranian nuclear infrastructure and personnel. How Iran is responding to the latest attack could pose an early challenge to the future Biden administration, which has said it will rejoin the nuclear deal that Mr Trump has rejected.


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