The Massive Ordnance Penetrator (Mop) is a precision-guided ‘bunker buster’ bomb - United States Air Force.

It is thought that there is only one weapon capable of blowing up Iran’s fabled nuclear mountain.

The 30,000lb GBU-57F/B Massive Ordnance Penetrator bomb, or “Mop”, has a thick steel outer casing that can penetrate 65 yards of concrete before exploding.

It might get some way through the half a mile of earth and rock thought to protect Iran’s most closely guarded nuclear secrets.

The Fordow facility, which sits in a deep valley between the Hasan Aqa and Furdu mountain peaks, is Iran’s most heavily fortified nuclear site.

It is believed to have some 3,000 sophisticated centrifuges spinning constantly to produce the weapons-grade uranium needed to manufacture an atomic bomb.

Since launching Operation Rising Lion on Friday, Israel has struck two Iranian nuclear facilities: Natanz, an underground enrichment plant, and the Isfahan plant.

According to Rafael Grossi, head of the UN’s nuclear watchdog, Natanz was “severely damaged if not destroyed altogether”, following the initial strikes last Friday. Mr Grossi also claimed that four buildings had been visibly damaged at Isfahan.

But Fordow, he said, remained unscathed, and attacking it posed a challenge.

With Israel’s 1981 attack on an overground Iraqi nuclear facility in mind, Iran buried Fordow’s two enrichment halls half a mile inside the mountains.

Israel doesn’t have any Mop munitions. But the US does.

A Mop would be dropped by an American B-2 stealth bomber, but it has never been used in actual warfare.

Israel has been blocked by the White House and Pentagon on multiple occasions from getting hold of its own high-powered “bunker buster”.

Israel is also not known to have any heavy bombers capable of delivering such a payload.

But this doesn’t mean the Israel Defence Forces (IDF) do not have a plan for disabling Fordow, or at least rendering the facility useless, as part of Operation Rising Lion.

“It is hard to imagine that Israel entered this fight with no plan at all to try to mitigate the threat posed by this facility,” Justin Crump, CEO of geopolitical risk firm Sibylline, told The Telegraph.

“The heart of Iran’s potential nuclear capability lies well buried at Fordow, with many regarding the use of multiple advanced US weapons being the only way truly to neutralise the facility.

“However, in recent months, there has been a growing view that Israel could instead use precision strikes on access points, ventilation shafts, and power supplies to at least heavily impact the use of the facility,” he added.

In practice, this more surgical approach could make it virtually impossible for Iran’s regime to keep its centrifuges spinning.

Coupled with the fact that the IDF killed at least nine of Tehran’s most prominent nuclear scientists, there are questions over whether the knowledge will exist to get them turning again.

Credit: USAF/WarLeaks

Despite the Israeli operation to destroy Iran’s nuclear capabilities, Fordow appears to have remained largely untouched.

The IDF has reportedly cooked up a number of strategies that it could use to render the facility useless, if a decision is taken to do so.

One plan previously presented to the Obama administration was using transport helicopters to drop Israeli commandos off at the site, let them fight their way in, before rigging it with explosives and bringing it down from the inside.

Hugely risky and potentially deadly for those involved, it would take a brave decision by Benjamin Netanyahu, Israel’s prime minister, to sanction such an attack, although similar tactics were used against a Hezbollah missile factory in Syria last year.

The most logical, and risk-adverse move given Israel’s aerial supremacy over Iran, would be to surgically immobilise the facility with strikes on its power supplies, access tunnels and ventilation shafts.

While it wouldn’t be entirely destroyed, Fordow would at least temporarily cease to be an operable part of Iran’s nuclear programme.

These are considered Israel’s strongest cards without Donald Trump sanctioning US involvement in attacks on the facility.

The US president has shown no signs of reversing a decades-long policy of permitting Israeli ownership, or at least use, of its bunker busters.

He has also made clear that he will steer clear of joining the military campaign. His fear of nuclear fallout is perhaps another reason for his hesitance to drop a bomb on Fordow: Such a strike could hurl an untold amount of contaminated earth and rock into the atmosphere.

The blast at the Chernobyl nuclear power plant in 1986, the world’s worst radioactive disaster, saw a radiation cloud spread for 77,000 square miles.

Mr Trump is said to have nuclear nightmares, stemming from his childhood during the Cold War, when schools would train their students to take cover under classroom desks.

He once cited a “nuclear winter” as the biggest threat to the world in a conversation about climate change with former prime minister Theresa May.

“It was a terrifying, nightmarish image seared into his mind,” Fiona Hill, a one-time foreign policy aide to the US President, previously said.

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What to know about the US bunker-buster bomb and how it could be used against Iran

Israel's attack on Iran aimed at destroying its nuclear program has raised speculation about whether the U.S. would use one of America's most powerful weapons to make that happen.

The Israeli government alleges Iran is close to making a nuclear weapon and the enriched uranium needed to do so is believed to be produced and kept inside its Fordo facility, deep inside a mountain in northwestern Iran.

One of the few weapons capable of damaging that facility and the centrifuges believed inside is the latest GBU-57 A/B Massive Ordnance Penetrator bomb -- known as the bunker buster.

It's 20-feet long, weighs 30,000 pounds and is capable of penetrating 200 feet deep inside a target and then exploding.

ABC News contributor and retired Col. Steve Ganyard said the bomb, which has never been used in combat, was designed specifically to be used against targets in Iran and elsewhere where facilities have been built underground and protected by thick concrete.

"You make sure you save them for places like Iran and North Korea because you don't need something that goes that deep" in normal circumstances, he said.

Ganyard noted that the bomb contains only 5,000 pounds of explosives designed to detonate once it reaches its subterranean target.

"It wouldn't be a massive blast. "It would penetrate the ground and shoot up some debris but it won’t be a massive cloud," he said.

No other military, including Israel, has access to the GBU-57 or the platform that can deliver the huge payload, the B-2 Spirit stealth bomber, which can carry two of the giant bombs.

The Air Force bombers are currently stationed in Whiteman Air Force Base Missouri and would take 15 hours to get to Iran if deployed, according to military experts.

Gaynard noted that the stealth capabilities of the B-2 would not be a factor in a hypothetical bunker buster mission in Iran as Israel's military has destroyed a major part of Iran's air defense systems.

Iranian officials have dismissed Israel's claims that it was working on a nuclear weapon.

Trump has issued warnings to Iran's leaders about its alleged plans to build a nuclear weapon and he has backed the Israeli government's ongoing attacks against Iranian targets.

He has remained noncommittal about possible U.S. involvement.

IAEA Director General Rafael Mariano Grossi issued a statement last week warning about the dangers of attacks on nuclear facilities, including the risk of nuclear contaminiation.

"Such attacks have serious implications for nuclear safety, security and safeguards, as well as regional and international peace and security," he said in a statement.