Deep below a mountain in Iran sits a once-secret uranium enrichment facility which now threatens to drag the United States into the Israel-Iran conflict.
There is one bomb – a so-called "bunker-buster" – which may be able to reach and destroy the facility.
But there is?only one country in the world which could drop it.
This is the Fordow Fuel Enrichment Plant, located near the city of Qom.
Aerial imagery shows the facility nestled in a mountain.
It is accessed via tunnels, evidenced by entrances visible from the surface.
The bulk of the nuclear facility is located under an estimated 90 metres of rock.
At its heart is a large hall which houses centrifuges, used to enrich uranium to certain percentages.
As if a mountain was not defence enough against air strikes – there are thought to be blast or debris traps near its entrances.
A low percentage of enrichment – about 3 to 5 per cent – is required for the uranium to be used in civilian settings, like a nuclear power plant.
A high level – generally about 90 per cent – is needed for use in modern atomic weapons.
Fordow’s location means conventional bombs, like those in Israel’s possession, would have little to no effect on the parts of the facility buried deep.
A secret base revealed
In 2009, then-US president Barack Obama stood alongside the leaders of France and the UK, and revealed Iran had been building a "covert uranium enrichment facility near Qom for several years".
He said a week earlier, Tehran had written to the UN’s nuclear watchdog, the International Atomic Energy Agency (IAEA), mentioning "a new enrichment facility, years after they had started its construction".
IAEA inspectors were allowed into the facility in late 2009, where they were shown two halls, according to a 2019 report by the Institute for Science and International Security (ISIS).
The think-tank is led by David Albright, a leading American physicist and nuclear weapons expert, who is also a former weapons inspector.
The report said one hall contained what one might expect for the enrichment of uranium for use in nuclear reactors, but the other hall was being stripped and modified at the time.
"These observations contributed to several inspectors, including ones who were experts in gas centrifuges, becoming suspicious that this hall was for the onward enrichment of uranium up to weapon-grade," the report, authored by Mr Albright and two others, read.
"Of course, Iran denied any such work," it said.
Under the 2015 Joint Comprehensive Plan of Action (JCPOA), colloquially known as the Iran deal, Fordow was allowed to operate as a nuclear physics and technology centre, but was banned from uranium enrichment and storing nuclear material at the site for 15 years.
“The US and allied negotiators were not able to convince Iran to shut down this site, even though it has no credible civilian nuclear justification,” the ISIS report said.
In 2018, during his first term, US President Donald Trump withdrew the US from the Iran deal, reportedly just days after a briefing from the Israeli prime minister on a daring raid in Tehran by Israel’s foreign intelligence service, Mossad.
A raid in the night
The raid on Iran’s "Atomic Archive", as it was labelled by Israel, has been extensively documented by the New York Times and Washington Post.
The publications describe a clandestine night raid in early 2018 on a nondescript warehouse in Tehran by agents of Mossad.
The agents torched their way into some of the 32 safes contained in the warehouse, after a two-year surveillance operation, stealing and smuggling out of the country tens of thousands of documents, and compact discs — containing memos, videos, and plans, relating to Iran's past nuclear research.
Israeli Prime Minister Benjamin Netanyahu announced the heist in April 2018 – presenting some of the documents in a televised address, arguing against the Iran deal.
Later, select media outlets were given access to some of the haul by Israeli officials, who decided what they could and couldn’t see.
At the time, Iran said the documents were fraudulent.
"Iran has always been clear that creating indiscriminate weapons of mass destruction is against what we stand for as a country and the notion that Iran would abandon any kind of sensitive information in some random warehouse in Tehran is laughably absurd," a statement from its UN mission in New York read.
But among the haul, was a picture showing support facilities for the Fordow nuclear facility, then named the Al Ghadir project site.
It also contained designs and diagrams for the underground portion of the project, according to the Institute for Science and International Security.
The ABC's 3D map of the facility, featured above, is based on these blueprints — which are understood to be the only publicly available layouts of the facility.
The IAEA says Iran stopped implementing all of the commitments it made as part of the Iran deal in February 2021 – including allowing daily access to Fordow, on request, for monitoring.
The watchdog has still been able to verify what's happening at Fordow at less-regular intervals.
Its latest report, released more than a week ago, said the facility is enriching uranium to 60 per cent — adding changes in its enrichment process had "significantly" increased the rate of production.
"It’s actually easier to go from an enrichment of 60 per cent to 90 per cent, than it is to get to that initial 60 per cent,” said nuclear physicist Kaitlin Cook in The Conversation.
"It's a fairly trivial last step to go 90 per cent, which is why people were alarmed," said Jeffrey Lewis, an expert in nuclear non-proliferation from the Middlebury Institute of International Studies.
"Fordow is not the largest enrichment facility, but it is the enrichment facility that we expected Iran to build as part of its nuclear weapons program.
"If your goal is to eliminate the nuclear program, you have to eliminate Fordow," he said.
Dan Shapiro, who was the US ambassador to Israel under Barack Obama until 2017, told ABC TV's 7.30, he believes Fordow needs to be destroyed.
"If it survives and continues to be a facility where they can enrich at 60 per cent and when they choose, to sprint to 90 per cent, this campaign will not have achieved its objective," he said.
"They [Iran] will remain capable and maybe even more motivated to produce a nuclear weapon at any time of their choosing."
Iran has long denied having a nuclear weapons program.
"Iran declared … quite a few times that … it does not have any nuclear, you know, just program in terms of military aspects," the Iranian Ambassador to Australia, Ahmad Sadeghi, told the ABC's David Speers, on Wednesday.
The massive ordnance penetrator
According to experts from the Royal United Services Institute, there is only one conventional weapon thought to be big enough to reach and destroy Fordow.
The GBU-57 Massive Ordnance Penetrator (MOP) is a behemoth.
It weighs more than 13 tonnes, stands six metres tall, and is specifically designed to "defeat hard and deeply buried targets", like bunkers and tunnels, according to a fact sheet from a US Department of Defense agency.
It is said to be able reach depths of up to 60 metres, before exploding.
"Multiple GBU-57/B impacts would almost certainly be required to reach the Fordow Fuel Enrichment Plant, with the second bomb impacting inside the hole made by the first," said Justin Bronk, an airpower specialist at RUSI.
The United States is the only country known to have this kind of bomb – and the only one with the aircraft approved to deliver it, the B-2 Spirit Stealth Bomber.
"While each B-2 can carry two GBU-57/Bs, such an attack would require redundancy since the weapons would have to function and be delivered perfectly to get down into the facility, and explode at the right depth to cause critical damage," Professor Bronk told ABC NEWS Verify.
Another bombing aircraft, the B-52, has dropped the bombs during testing.
B-2 bombers operate out of a US Air Force base in Missouri.
But the uniquely shaped aircraft have, as recently as April, been seen at an air base on the island of Diego Garcia, in the Indian Ocean.
That same month, US media reported the B-2s were being stationed there in a display of power to countries like Iran.
Whether they take off from the US, or the Indian Ocean, the stealth bombers’ range limitations mean they might need to refuel while in the air on any mission to Iran.
According to Reuters, AirNav systems, a flight tracking website, said more than 31 US Air Force refuelling aircraft – primarily KC-135s and KC-46s – left the United States on Sunday.
It said the flights had landed in Europe.
The decision to bomb, or not, will ultimately be decided by the US commander in chief, Donald Trump.
"I may do it. I may not do it," he said on Wednesday.
The White House said on Friday that Trump would decide on whether to intervene in the conflict, or not, within two weeks.
If a bombing raid is given the green light by the US president – there is no guarantee it will work.
But in a 2024 podcast, David Albright said he believed Fordow was "more vulnerable" than people think.
"We have the building designs, it’s in the nuclear archive," he told Arms Control Poseur.
"Israel has even more of those designs, they know exactly how the tunnels go, where they start, how they zig and zag, where the ventilation system is, the power supplies.
"You don’t have to bring down the roof of the enrichment hall to put that facility out of operation for a long time," he said.
Notes about data used in this story:
The satellite image used in the 3D model is from Planet Labs. Elevation data is from the Shuttle Radar Topography Mission (SRTM) and map boundaries are from Natural Earth.
