To Israeli military planners, it is akin to Mount Doom: a tightly guarded nuclear enrichment plant, buried half a kilometre beneath a mountain, which is ringed by air defences and symbolically situated near the ancient religious city of Qom.
To Tehran, the Fordow facility symbolises its desire to safeguard its nuclear programme, designed to survive a full-frontal attack, with enough centrifuges and highly enriched uranium intact to potentially produce a nuclear weapon, or “break out”.
Buried under hard rock and encased in reinforced concrete that puts it beyond the destructive reach of any of Israel’s publicly known weapons, it is also a symbol of Iran’s strategic anxiety.
“Fordow is the be-all and end-all of Iran’s nuclear operation,” said Behnam Ben Taleblu, a senior fellow at the Foundation for Defense of Democracies, a US think-tank.
On Saturday, President Donald Trump announced that US planes had bombed Fordow, alongside other nuclear sites at Natanz and Isfahan, saying that the facilities had been “completely and totally obliterated”.
Iran confirmed the strikes, though the extent of the damage was yet to be verified on Sunday morning.
In global terms, Fordow is not a uniquely protected facility. Every major military power with a nuclear programme has similar underground military bunkers that have inspired countless spy thrillers and conspiracy theories.
Raven Rock in the US, the so-called “Underground Pentagon”, is built into a Pennsylvania mountain. Russia’s secretive Mount Yamantau is thought to house a large nuclear weapons facility.
The same is true of the North Korean underground missile bases built into the mountains, while China’s Longpo naval base includes an underground facility for nuclear submarines accessed via tunnels.
Fordow, however, is the only major underground military base that has ever been directly attacked — a precedent which shows the extraordinary risks that Israeli Prime Minister Benjamin Netanyahu has taken by authorising the Israeli attacks this week.
Iranian officials have long denied seeking a bomb, and the most recent threat assessment issued by the US intelligence community this year concluded that Iran had not restarted the nuclear weapons programme that it suspended under international pressure in 2003.
But were Tehran to take that path, the ISIS estimates Fordow could convert Iran’s entire stock of high enriched uranium — assessed to be 408kg by IAEA inspectors in May — to produce in just three weeks enough weapons grade uranium to make nine nuclear weapons.
“Iran could produce its first quantity of 25kg of [weapons-grade uranium] in Fordow in as little as two to three days,” ISIS has warned.
The differences between Fordow and Natanz encapsulate much of Tehran’s nuclear history — as well as the multilateral efforts taken to curb its enrichment efforts and thus pre-empt the kind of attack that Israel launched this week.
Following public revelations about a secret facility, Natanz was eventually declared by Iran to the UN in 2003. While the sprawling industrial complex contains as many as 16,000 centrifuges, the design is for mass-scale uranium enrichment at lower levels.
This, combined with regular UN inspection, made it more suitable for civilian-nuclear use. Natanz’s underground enrichment plant is also buried only about 20m underground.
By contrast, what makes Fordow stand out is a geologic robustness that makes its centrifuge halls effectively impenetrable to air-delivered conventional bombs.
The US’s giant bunker-buster bombs are considered the only weapons capable of destroying Fordow. On Saturday, multiple US B-2 bomber aircraft — which are used to carry bunker-busters — travelled from the US to the Pacific.
Built in secret, Fordow’s existence was publicly unveiled in September 2009 during a moment of high drama when US, British and French officials declassified intelligence showing that Iran had covertly built a secret plant deep inside a mountain that was “inconsistent with a peaceful programme”.
The finding, which cemented what Britain’s then-prime minister Gordon Brown called Iran’s “serial deception”, was so dramatic that it led to a rare Russian rebuke of Iran and a warning from China.
Iran stood firm at the time. “What we did was completely legal,” said then president Mahmoud Ahmadi-Nejad, adding: “What business is it of yours to tell us what to do?”
Even so, Fordow came to lie at the centre of subsequent international attempts to curb Iran’s nuclear programme.
It led to increased UN sanctions and was at the heart of a multilateral 2015 deal, known as the Joint Comprehensive Plan of Action, between Iran and world powers including the US, UK, France, Russia, China and Germany
In return for sanctions relief Iran agreed, among other measures, to convert the facility into a research centre, cap the number of centrifuges there, halt uranium enrichment for 15 years, and allow for enhanced monitoring by international inspectors.
The US withdrew from JCPOA in 2018 during the first presidency of Donald Trump, and, since then, Iran has moved to enrich more uranium.
Following a 2021 explosion at Natanz, which Iran blamed on Israel and that damaged its enrichment capacity, Tehran set the Fordow centrifuges whirring instead. These began to convert Iran’s stock of low-enriched uranium to 60 per cent purity, from which it is possible to produce weapons-grade uranium within days.
Analysts believe that Fordow, if it is not destroyed, could form the centre of Iranian efforts to “break out”. The country could withdraw from the nuclear non-proliferation treaty, halt co-operation with the IAEA, and quickly build a nuclear bomb.
Iran has previously threatened just such a response if its nuclear facilities were ever targeted, although such a move could draw the US military into the Israeli campaign.
Adding to the risks is that Fordow is not the only ultra-secure facility that Iran can fall back on. Tehran has recently been building an even deeper and better-protected facility into Kūh-e Kolang Gaz Lā, also known as Pickaxe mountain, a few kilometres south of Natanz.
While Fordow is thought to have two tunnel entrances, Pickaxe has at least four, making it harder to seal off entrances by bombing. Its underground halls also have more floorspace.
Some fear the facility, which Iran has so far barred the IAEA from inspecting, could even be used to assemble a nuclear weapon while Iran was under attack.
“A key question is whether Iran will, or maybe already has, secreted fissile material into Pickaxe, or some other unknown facility,” said the FDD’s Ben Taleblu.
Animation by Gaku Ito, graphic illustration by Ian Bott
Letter in response to this article:
Will Israel’s ‘secret’ facility be next in the spotlight? / From Nicky Browne, Broadstairs, Kent, UK