Deep beneath a mountain, hundreds of centrifuges spin, enriching Iran’s uranium that Israel suspects is destined for a nuclear weapon.

The Fordow plant is protected by tonnes upon tonnes of dirt and rock, far away from prying eyes – and foreign missiles.

But as Israeli warplanes fly unchecked above Tehran, with much of the Islamic Republic’s air defences turned to smoking ruins on the ground, attention has moved to the secretive facility.

Some say only the American B-2 stealth bomber and its massive payload could breach the so-called “nuclear mountain”, while others argue troops on the ground might be able to infiltrate its corridors. Or maybe it is simply impossible, short of a nuclear strike.

Iran has repeatedly denied that it is seeking a nuclear weapon and the head of the UN’s nuclear watchdog said in June that it has no proof of a “systematic effort to move into a nuclear weapon”.

What is the Fordow facility?

The Fordow enrichment plant is one of three key pieces of nuclear infrastructure in Iran – the others being the Natanz enrichment plant and research facilities in Isfahan.

It is thought to be buried around 80m deep into the side of the mountain. It was previously protected by Iranian and Russian surface-to-air missile systems, but these may have wholly or partially knocked out during Israel’s recent attacks.

Construction is believed to have started in around 2006 and it first became operational in 2009 – the same year Tehran publicly acknowledged its existence.

In November 2020, it was believed there were 1,057 centrifuges at Fordow. These are used to separate isotopes and increase the concentration of uranium-235, needed for nuclear fuel and weapons.

In 2023, the International Atomic Energy Agency (IAEA) – the nuclear watchdog – found uranium particles enriched to 83.7% purity – near the 90% needed for a bomb – at Fordow, the only Iranian facility where this has been found.

In June 2024, the Washington Post reported on a major expansion at Fordow, with nearly 1,400 new centrifuges earmarked for the subterranean facility.

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Will Israel try to destroy Fordow?

Israel has made no secret of its desire to cripple or remove Iran’s nuclear programme, describing it as an existential threat.

There is much that remains elsewhere in Iran that is capable of producing and using nuclear material.

“But of course the real big piece remains at Fordow still and this has been in the headlines quite a bit,” says Dr Alexander Bollfrass, an expert on nuclear weapons from the International Institute for Strategic Studies (IISS) thinktank.

There is also the chance that an increased focus on diplomacy brings the war to an end before the IDF can make a run at Fordow.

Could bunker buster bombs be used?

There has been a lot of talk about bunker buster bombs. These are munitions that explode twice – once to breach the ground surface and again once the bomb has burrowed down to a certain depth.

The Israelis used 60 to 80 of them in the strike that killed Hezbollah leader Hassan Nasrallah in September last year, according to Martin “Sammy” Sampson, a former air marshal and executive director at the IISS.

But Nasrallah was only 10-15m underground, Mr Sampson said, while Fordow is believed to be 80m beneath the surface.

“An awful lot of planes would be in the same place for an awful long time” to drop enough bombs to have a chance of getting to the buried facility, he added.

There is also the possibility that the US, which operates the much more powerful GBU-57 bomb, could assist with any operation at Fordow.

“My sense is that it would still take multiple strikes,” Mr Sampson said, putting it in “more and more unknown territory”.

“It would be pretty disastrous… if you put 400 planes over the top of Fordow, or you put the might of the US over Fordow, and it survived.”

Israel’s ‘contingencies’ for dealing with Fordow

Israel has suggested that it could destroy or cripple Fordow without using bombs dropped from the air.

Speaking to Sky’s Yalda Hakim earlier this week, former Mossad director of intelligence Zohar Palti said it was “much easier for the Americans to do it”, possibly referring to the GBU-57.

“But as you see, we know how to run things alone,” he added. “And if we need to do some other stuff alone, we will do it.”

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Israel’s ambassador to the US, Yechiel Leiter, said last weekend that Israel has “a number of contingencies… which will enable us to deal with Fordow”.

“Not everything is a matter of, you know, taking to the skies and bombing from afar,” he told ABC News.

There has been talk of using special forces to raid the facility on the ground, but that has its downsides as well.

“This would be an incredibly high risk mission if you were to do something on the ground,” said Mr Sampson.

There is also the possibility Israel could replicate what happened at the Natanz enrichment plant, where the IAEA said 15,000 centrifuges were likely destroyed in the IDF bombardment of Iran.

This was possibly due to an Israeli airstrike disrupting the power supply to the centrifuges, rather than actual physical damage to the centrifuge hall, according to the nuclear watchdog.

Regime change in Iran is “unacceptable” and the assassination of the country’s supreme leader would “open the Pandora’s box”, the Kremlin has said.

In a rare interview with a foreign media organisation, Kremlin spokesman Dmitry Peskov told Sky News that Russia would react “very negatively” if Ayatollah Ali Khamenei was killed.

The comments came as US President Donald Trump said he will decide within two weeks whether America will join Israel’s military campaign against Tehran, after earlier speculating on social media about killing the Iranian leader.

“The situation is extremely tense and is dangerous not only for the region but globally,” Mr Peskov said in an interview at the Constantine Palace in Saint Petersburg.

“An enlargement of the composition of the participants of the conflict is potentially even more dangerous.

“It will lead only to another circle of confrontation and escalation of tension in the region.”

They are the Kremlin’s strongest comments yet regarding the Israel-Iran conflict, which has stoked fears in Moscow that it could be on the verge of losing its closest ally in the Middle East.

Russia has deepened its ties with Iran since invading Ukraine, and the two countries signed a strategic partnership in January.

“[Regime change in Iran] is unimaginable. It should be unacceptable, even talking about that should be unacceptable for everyone,” Mr Peskov said, in a thinly veiled reference to Washington.

But Mr Peskov refused to be drawn on what action Russia would take if Khamenei was killed, saying instead it would trigger action “from inside Iran”.

“It would lead to the birth of extremist moods inside Iran and those who are speaking about [killing Khamenei], they should keep it in mind. They will open the Pandora’s box.”

Vladimir Putin’s offers to mediate an end to the conflict have so far been rejected by Mr Trump, who said on Wednesday that he told the Russian president to “mediate your own [conflict]”, in reference to Russia’s war against Ukraine.

Mr Peskov denied the American president’s words were insulting, adding: “Everyone has a different language.

“President Trump has his own unique way of speaking and his unique language. We are quite tolerant and expect everyone to be tolerant of us.”

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The Trump administration’s own mediation efforts to end the war in Ukraine have failed to yield any major breakthroughs, despite two rounds of direct talks between Moscow and Kyiv.

Moscow has stepped up its aerial bombardment of Ukraine in recent weeks and continues to reject Volodymyr Zelenskyy’s calls for a 30-day ceasefire.

“Now we have a strategic advantage. Why should we lose it? We are not going to lose it. We are going further. We’re advancing and we’ll continue to advance,” Mr Peskov said.

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Russia has previously said it would only commit to a ceasefire if Kyiv stops receiving foreign military support, fearing that a pause in the fighting would offer Ukraine a chance to rearm and regroup its forces.

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Asked if Moscow could commit to not using a ceasefire in the same way, Mr Peskov said: “A ceasefire is a ceasefire, and you stop.

“But America is not saying that ‘we’ll quit any supplies’. Britain is not saying that as well. France is not saying that as well. This is the problem.”

China has criticised a British warship’s passage through the Taiwan Strait as a deliberate move to “cause trouble”.

The Royal Navy said its patrol vessel HMS Spey was conducting a routine navigation through the contested waterway on Wednesday as part of a long-planned deployment in compliance with international law.

In response, the Eastern Theatre Command of China’s People’s Liberation Army (PLA) said the exercise was “public hyping”, adding that its forces followed and monitored the ship.

“The British side’s remarks distort legal principles and mislead the public; its actions deliberately cause trouble and disrupt things, undermining peace and stability in the Taiwan Strait,” it said in a statement on Friday.

“Troops in the theatre are on high alert at all times and will resolutely counter all threats and provocations.”

The last time a British warship sailed through the strait was in 2021, when HMS Richmond was deployed in the East China Sea en route to Vietnam.

The strait is contested between Taiwan and China, which split in 1949.

Today, China views Taiwan as a breakaway province – with which it promises to one day reunify, and has not ruled out the use of force to do so – and regards the waterway as its own territory.

Taiwan, the US, and other Western powers regard the strait as international waters.

US navy ships sail through the strait around once every two months, sometimes accompanied by those of allied nations.

Responding to HMS Spey’s exercise, Taiwan’s foreign ministry said it “welcomes and affirms the British side once again taking concrete actions to defend the freedom of navigation in the Taiwan Strait”.

China has also carried out several military drills across the waterway, with exercises in October involving its army, navy and rocket forces. Beijing called it a “stern warning to the separatist acts of Taiwan independence forces” at the time.

Read more from Sky News:
Kremlin: Regime change in Iran is ‘unacceptable’
Inside Britain’s largest nuclear weapons site
Father, 27, shot dead in ‘mistaken identity’

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It comes as Taiwanese President Lai Ching-te on Thursday ordered defence and security units to step up their monitoring and intelligence efforts in response to China’s military activities.

Taiwan’s defence ministry also reported another spike in Chinese movements close to the island over the previous 24 hours, involving 50 aircraft, concentrated in the strait and the top part of the South China Sea.