For Israeli military planners, it is like Mount Doom: a heavily guarded nuclear enrichment plant, buried half a kilometer beneath a mountain, surrounded by air defenses, and symbolically located near the ancient religious city of Qom.
For Tehran, the Fordo facility symbolizes its aspiration to preserve its nuclear program, designed to survive a frontal attack, with enough centrifuges and highly enriched uranium intact to potentially produce nuclear weapons.
Buried in hard rock and lined with reinforced concrete, which places it beyond the reach of any publicly known Israeli weapon, it is also a symbol of Iran's strategic anxiety.
“Fordo is the alpha and omega of Iran’s nuclear operation,” said Behnam Ben Taleblu, a senior fellow at the US-based Foundation for Defense of Democracies.
Iran said on Saturday that Fordow had been attacked, the semi-official ISNA news agency reported, citing the Atomic Energy Organization of Iran, although damage was limited.
In contrast, Israel succeeded in destroying Iran's larger above-ground uranium enrichment facility at Natanz, the head of the UN nuclear watchdog, Rafael Grossi, told the Security Council on Friday.
Its underground centrifuge halls may have become unusable due to severe damage to Natanz's electrical infrastructure, according to an analysis of open-source satellite imagery by the Institute for Science and International Security.
"Without US help, Fordow will be a big challenge. It's heavily fortified and deep in the mountain. I'm not sure how much damage we can do there," said Danny Citrinovich, an Iran expert at the Institute for National Security Studies in Tel Aviv.
“Iran is not yet close to ground zero (the complete destruction of its nuclear program)… They still have significant capabilities,” Citrinovich added, noting that Fordow could be the most difficult, and perhaps the last, target in Israel’s air campaign.
On a global scale, Fordow is not the only specially protected facility. All major military powers with nuclear programs have similar underground military bunkers, which have inspired countless spy thrillers and conspiracy theories.
Raven Rock in the US, the so-called “underground Pentagon”, is built inside a mountain in Pennsylvania. Russia’s mysterious Yamantau mountain is believed to house a large nuclear weapons facility.
The same is true of North Korea's underground missile bases, dug into the mountains, while China's Longpo naval base contains an underground facility for nuclear submarines, accessed through tunnels.
However, Fordow is the only major underground military base to have ever been directly attacked - a precedent that testifies to the extraordinary risks Israeli Prime Minister Benjamin Netanyahu took in authorizing the Israeli strikes last week.
Iranian officials have long denied that they are pursuing a nuclear bomb, and the most recent threat assessment by the US intelligence community, released this year, concluded that Iran has not resumed a nuclear weapons program that it suspended in 2003 under international pressure.
However, if Tehran were to decide to go that route, the Institute for Science and International Security estimates that Fordo could process Iran's entire stockpile of highly enriched uranium - which, according to an IAEA inspector's estimate in May, amounted to 408 kg - and produce enough uranium to make nine nuclear bombs in just three weeks.
"Iran could produce the first 25 kg (of military-grade uranium) at Fordow in just two to three days," the Institute for Science and International Security warned.
The differences between Fordow and Natanz largely reflect the entire history of Iran's nuclear program, as well as the multilateral international efforts to curb its uranium enrichment and thereby prevent an attack like the ones Israel is carrying out these days.
After the existence of the secret facility was revealed, Iran reported Natanz to the United Nations in 2003. Although the vast industrial complex contains as many as 16.000 centrifuges, its design is intended for mass enrichment of uranium at lower levels.
This, combined with regular UN inspections, has made Natanz more suitable for civilian nuclear use. Its underground enrichment facility is also buried at a depth of only about 20 meters.
In contrast, what sets Fordow apart is its geological resistance, which makes the centrifuge halls virtually impenetrable to conventional air-delivered bombs. This could even include the giant American bunker-busting bomb capable of penetrating 60 meters of concrete.
Built in secret, Fordow's existence was revealed in September 2009, in a moment of great drama, when US, British and French officials declassified intelligence showing that Iran had secretly built a facility deep in the mountains that was "incompatible with a peaceful program."
The revelation, which solidified what then-British Prime Minister Gordon Brown called Iran's "serial deception," was so powerful that it prompted a rare condemnation of Iran from Russia and a warning from China.
Iran stood by its stance at the time. “What we did was completely legal,” said then-President Mahmoud Ahmadinejad, adding: “What does that have to do with you telling us what to do?”
Despite this, Fordow became a focal point of subsequent international efforts to curb Iran's nuclear program.
This led to increased UN sanctions and was at the very core of the 2015 multilateral agreement, known as the Joint Comprehensive Plan of Action (JCPOA), which Iran signed with world powers, including the US, UK, France, Russia, China and Germany.
In exchange for the lifting of sanctions, Iran agreed, among other things, to convert the facility into a research center, limit the number of centrifuges, suspend uranium enrichment for 15 years, and allow increased monitoring by international inspectors.
The United States withdrew from the JCPOA in 2018, during Donald Trump's first presidential term, and since then, Iran has begun enriching larger amounts of uranium.
Following the 2021 explosion at Natanz, which Iran blamed on Israel and damaged enrichment facilities, Tehran started up centrifuges at Fordow. They began converting Iran's stockpile of low-enriched uranium into 60 percent-pure uranium - which can be converted into weapons-grade uranium in just a few days.
Analysts believe that Fordow, if not destroyed by Israeli strikes, could become the center of Iran's so-called "breakthrough" efforts to build a nuclear bomb. The country could withdraw from the Nuclear Non-Proliferation Treaty, suspend cooperation with the International Atomic Energy Agency (IAEA), and produce a nuclear bomb in the short term.
Iran has previously threatened just such a response if its nuclear facilities were attacked, although such a move could also draw the US military into the Israeli campaign.
An additional risk is that Fordow is not the only ultra-secure facility Iran can rely on. Tehran has recently begun building an even deeper and more protected facility inside the Kuh-e Kolang Gaz La mountain, also known as “Mount Kramp,” a few kilometers south of Natanz.
While Fordo is thought to have two tunnel entrances, Kramp Mountain has at least four, making it more difficult to block the entrances with bombing. Its underground halls also have a larger surface area.
Some fear that the facility, which Iran has so far refused to allow IAEA inspectors access to, could even be used to assemble nuclear weapons while the country is under attack.
"The key question is whether Iran will bring, or perhaps has already brought, fissile material to 'Kramp', or to some other, still unknown facility," said Behnam Ben Taleblu of the Foundation for Defense of Democracies.
Translation: NB
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