Hidden among the mountains in central Iran, work has been continuing on a mysterious underground facility believed to be buried beyond the range of US "bunker buster" bombs.
The site, known as Pickaxe Mountain, or Kuh-e Kolang Gaz La, has never been accessed by international nuclear inspectors, and its exact purpose remains unclear.
Analysts monitoring its development via satellite imagery have witnessed security walls growing, spoil piles expanding and tunnel entrances being reinforced as engineers dig deeper into the mountain.
"We don't have internal schematics to really judge what the inside will look like," says Spencer Faragasso, a senior research fellow at the US Institute for Science and International Security.
"But given the size of the spoil piles, the amount of construction they're doing, it wouldn't be incomprehensible to see them establish an enrichment facility inside it."
Located near the peak of the Zagros Mountains, the site is just 1.6 kilometres south of Natanz, which was Iran's main uranium enrichment facility.
But Pickaxe Mountain was not affected when Natanz and two other key Iranian nuclear facilities — Fordow and Isfahan — were targeted in US strikes that aimed to disrupt Tehran from potentially developing nuclear bombs.
[map — with pickaxe location]US President Donald Trump said the three sites were "obliterated" in the June 2025 attacks, but has renewed demands for Iran to make a deal over its nuclear program or face fresh strikes that would be "far worse".
Negotiators from both countries held indirect talks in Oman on Friday, with Iran's top diplomat striking a cautiously optimistic note after their conclusion.
However, the US delegation, led by special envoy Steve Witkoff and Mr Trump's son-in-law, Jared Kushner, did not offer any immediate comment.
Recent assessments show Tehran's nuclear program was severely damaged by the US during the 12-day war between Iran and Israel, but it could be built up again.
And satellite imagery revealed Pickaxe Mountain could be a "potential candidate" for new uranium enrichment activities.
Iran's nuclear ambitions
Soon before the US sent a group of B-52 bombers into Iranian airspace to carry out Operation Midnight Hammer, the International Atomic Energy Agency (IAEA) raised "serious concern" about Iran's nuclear program.
The UN nuclear watchdog assessed Iran had amassed 408.6 kilograms of uranium enriched up to 60 per cent, well on its way to the concentration required for a nuclear weapon.
Enriched uranium is key to developing nuclear bombs. Once enriched to 90 per cent, it is deemed weapons-grade.
The Institute for Science and International Security (ISIS), a Washington-based non-profit long focused on Iran's nuclear program, said Tehran could have converted its stock at the Fordow enrichment plant into at least nine nuclear weapons within three weeks.
Tehran has insisted for decades its nuclear program was peaceful, intended as a civilian program to meet energy needs.
A low percentage of enrichment — about 3 to 5 per cent — is required for uranium to be used in civilian settings, such as a nuclear power plant.
Satellite imagery shows 'extensive' damage
On June 22 last year, the US swooped in and struck Fordow, Natanz and Isfahan with 13,000kg "bunker buster" bombs capable of penetrating 61 metres of earth before exploding.
Mr Trump deemed the operation "a spectacular military success", although a leaked Pentagon intelligence assessment at the time claimed Iran's nuclear program was only set back by a few months.
The UN nuclear watchdog has not been given access to the sites since the attacks, so analysts have relied on satellite imagery to assess the extent of the damage.
The US Center for Strategic and International Studies (CSIS) recently determined US and Israeli attacks last year "inflicted significant damage on Iran's nuclear program by destroying key infrastructure and human capital".
Similar assessments were made by ISIS, which stated in a November report that "overall, the damage caused by air strikes to numerous nuclear sites was extensive and, in many cases, catastrophic".
Fordow, about 95 kilometres south-west of the capital, Tehran, housed a hardened enrichment site under a mountain.
Mr Trump considered it to be the "primary site", which analysts believed was Iran's deepest and most highly protected facility.
Natanz, a mix of above- and below-ground laboratories, was responsible for the majority of Iran's uranium enrichment.
The facility outside the city of Isfahan was mainly known for producing the uranium gas that is fed into centrifuges.
The tube-shaped centrifuges, arranged in cascades of dozens of machines, rapidly spin uranium gas to enrich it.
Mr Faragasso, senior research fellow at ISIS, said Iran did not have an enrichment program currently.
"Iran has no capability to create uranium hexafluoride — that's what you actually use to enrich uranium to higher levels," he told the ABC.
"And their centrifuge manufacturing and assembly has also largely been eliminated by Israeli strikes on a number of different areas."
Both ISIS and CSIS reports noted there had been little to no progress made to rehabilitate the damaged sites, but Pickaxe Mountain had caught their attention.
Its tunnels are estimated to be concealed between 79 and 100 metres into the earth, potentially deeper than the Fordow facility, "raising significant concern".
Movement at Pickaxe Mountain
Mr Faragasso said his team had been monitoring Pickaxe Mountain since construction began in 2020.
The think tank is led by David Albright, a leading American physicist and nuclear weapons expert, who is also a former weapons inspector.
In 2020, Iran declared it to be a centrifuge assembly facility.
But the scope of the work showed it could be "a potential candidate site for any Iranian reconstitution of its centrifuge program, from component production to assembly to enrichment," according to ISIS analysis.
The site had been heavily fortified to protect against attacks, and comprised underground hallways deep and vast enough to house an enrichment plant.
"Iran would be missing an opportunity to put an enrichment facility in there, given their experience during the war," Mr Faragasso said.
"They would want something that's more protected, that's harder to penetrate and destroy and this would be a candidate for that.
"We don't have confirmation of those things, but these are things we continue to watch."
IAEA director-general Rafael Grossi asked Iran about activities at Pickaxe Mountain.
"It's none of your business," was reportedly Tehran's response.
The Center for Strategic and International Studies said it was difficult to definitively assess the full purpose of Pickaxe Mountain from imagery alone.
But the construction of a large underground facility so close to Natanz was "highly suspicious".
Construction at the site has sped up since June last year, and an associated tunnel complex built in 2007 was recently encased in concrete slabs.
"Why would you go about hardening a tunnel that you're not going to use? It made us really think, did they move something in there?" Mr Faragasso said.
Where are the uranium stockpiles?
Iran's stockpiles of 60 per cent enriched uranium remain missing.
Trucks observed outside Fordow and Isfahan before and after the US strikes suggested Iran may have moved the material.
But the IAEA director said there was a "general understanding" the enriched uranium was likely still buried under the damaged facilities.
"We need to go back there and to confirm that the material is there and it's not being diverted to any other use," Mr Grossi said in October.
"This is very, very important."
Images from Planet Labs PBC issued last week show roofs have been built over two damaged buildings at the Isfahan and Natanz facilities, the first major activity noticeable by satellite since last June.
Andrea Stricker, who studies Iran for the Washington-based Foundation for Defense of Democracies, which has been declared a terrorist organisation by Tehran, said the roofs were likely intended to block satellites from capturing what was happening on the ground.
It could allow Iran to "assess whether key assets — such as limited stocks of highly enriched uranium — survived the strikes", she told the Associated Press.
"They want to be able to get at any recovered assets they can get to without Israel or the United States seeing what survived."
Mr Faragasso said Iran appeared to be concealing "salvaging operations".
"If they're trying to salvage equipment, it raises questions about what they're trying to do with this equipment," he said.
"We're dealing with a country that is absolutely determined to do everything it can to maintain the survival of its enrichment regime.
"Given enough time, energy and investment, they can reconstitute what they lost."
Relations between Washington and Tehran have been tense since Mr Trump's first term, when he withdrew the US from a 2015 international agreement limiting Iran's nuclear program.
Even after the Oman talks, Mr Faragasso said there were still so many unknowns about the extent of Iran's nuclear activities.
"This is why the IAEA needs to be able to do its job and Iran needs to come clean about what it's doing and provide a complete and full declaration of its activities, especially at the Pickaxe Mountain site," he said.
