SINGAPORE: Since Israel launched strikes against Iran on Jun 13, more than 200 have been killed, including top generals and scientists.

Israel described its action as a "preemptive strike" to prevent its longtime enemy from building nuclear bombs. It has repeatedly accused Iran of seeking a nuclear weapon, an allegation the latter has denied. 

With the two sides exchanging fire for six days now, attention has turned to the Fordow nuclear plant buried deep under a mountain in Iran. 

Reports say only one weapon has the capabilities to destroy it: The United States' GBU-57 Massive Ordnance Penetrator, known as a bunker-buster bomb. 

What is a bunker-buster bomb?

As its name suggests, the GBU-57 is designed to attack hard and deeply buried targets such as bunkers and tunnels.

The US air force said the GPS-guided weapon is capable of penetrating up to around 61m underground before exploding.

It is about 6.2m-long and weighs approximately 13,600kg.

The bomb's warhead is said to contain about 2,400kg of explosives - specifically the AFX-757 and PBXN-114 types - according to defence analysis website The War Zone. 

The warhead case is made from a special high-performance steel alloy, and its design allows for a large explosive payload while maintaining integrity.

At present, only the US owns the GBU-57 bomb.

How is the bomb deployed?

The B-2 Spirit is the only aircraft programmed to carry the bunker-buster bombs.

Produced by American aerospace and defence company Northrop Grumman, the jet is also only flown by the US air force.

The aircraft can hold two of these bombs, one in each bay.

US defence officials described the B-2 Spirit as a multi-role bomber capable of delivering both conventional and nuclear weapons. 

It has characteristics that allow it to penetrate sophisticated defences and strike heavily defended targets, without the enemy detecting its presence.

Has the bomb been used before?

There are no reports of the bomb having been deployed in combat. 

The US began developing it in the 2000s, with Boeing awarded a contract for the Massive Ordnance Penetrator programme in 2004. 

Since then, all known deployments of the GBU-57 have been for testing or training. 

Test drops were conducted in 2014, 2015 and 2016, according to aerospace and defence publication Air and Space Forces Magazine.

In 2017, several B-2 aircrafts completed four test drops at the White Sands missile range, the largest military base in the US. 

How does the GBU-57 compare with other bunker-busters?

Just going by weight, it has an edge over similar munitions.

To compare, the US' GBU-43/B Massive Ordnance Air Blast Bomb is about 9m long and weighs around 9,800kg. 

This used to be referred to as the "mother of all bombs", and was used against Islamic State militants in Afghanistan in 2017.

Unlike the GBU-57, it detonates shortly before hitting the surface - no ground penetration is involved. 

This bomb is filled with close to 8,500kg of explosives - significantly more than the GBU-57.

Other bunker-buster bombs in the US arsenal include the GBU‑28 and the BLU‑109.

The GBU-28 is a laser-guided weapon, weighing nearly 2,300kg. It can penetrate up to 6m of concrete.

The BLU-109 is roughly 900kg and like the GBU-57, is a penetration weapon that can be used against bunkers and aircraft shelters.

It can break through around 1.2m to 1.8m of reinforced concrete, and packs about 240kg of explosives, as reported by TWZ.

The latter two bombs have also been used in combat. The GBU-28 was used to destroy the main Iraqi command and control bunker during the Gulf War in 1991; while the US delivered 100 BLU-109 bombs to Israel which were then believed to be used in a strike that killed Hezbollah chief Hassan Nasrallah.

Russia too has its own bunker-buster bomb: the 1,500kg-FAB-1500. This is also precision-guided but is only able to penetrate to a depth of 20m.

Why is the GBU-57 the only weapon that can destroy Fordow? 

The Fordow facility sits between 80m and 90m below the peak of a mountain. 

This gives it strategic defensive capability against conventional attacks, and against anything in the Israeli arsenal, research assistant at the National University of Singapore's (NUS) Middle East Institute Damien Tan told CNA. 

"The GBU-57 is the only conventional weapon that has a chance of causing damage to Fordow," said Mr Tan. 

And this can only be delivered by the US, and its B-2 Spirit bomber. 

In theory, Israel could place the GBU-57 bombs on, for instance, its C-130 Hercules transport aircraft. 

But that plane is slow, flies at low altitude and is not equipped for effective targeting, making this option "strategically unsound", said Mr Tan. 

Why is Fordow so important?

The Fordow site lies in the city of Qom, around 95km southwest of Iranian capital Tehran. 

The existence of the Fordow fuel enrichment plant was first made known in 2009. Then-US President Barack Obama said that the size and configuration of the facility was "inconsistent" with claims that it was to be used for a peaceful, civilian nuclear energy programme. 

Fears of Iran developing nuclear weapons eventually led to the signing of a Joint Comprehensive Plan of Action in 2015. Under this deal, Iran was required to remove two-thirds of the centrifuges inside the facility, along with all nuclear material. The site was also barred from nuclear work. 

But President Donald Trump pulled the US out of the deal in 2018.

Iran subsequently restarted its programme and began producing 60 per cent enriched uranium.

In 2023, the International Atomic Energy Agency collected samples from Fordow that found uranium enriched to 83.7 per cent - very close to weapons-grade.

Iran's only other known underground nuclear enrichment site is the Natanz plant outside Tehran. Natanz sits around 8m below ground, and has already been targeted by Israeli air strikes.

NUS' Mr Tan said: "If Fordow is not completely destroyed, work there could resume in a matter of months, putting Iran back on a path towards a bomb in a short time if it pursues the option - a possibility with even odds, given the failure of its conventional deterrence (so far)."

Yet besides those publicly known, Iran may also possess other secret enrichment sites which would potentially allow it to continue developing nuclear capabilities. 

While targeting Fordow might delay Iran's nuclear progress, it would not eliminate the underlying threat, said Mr Tan. 

"This is why some have charged that Israel is pursuing regime change in addition to striking Fordow, which could signal the next step up the escalation ladder," he added. 

What are the challenges in trying to attack Fordow?

Given how deeply buried Fordow is, a single strike would not be enough to take down the facility, said Mr Tan. If the US decides to enter the war, it would have to use several GBU-57s, released in waves, to ensure Fordow is completely destroyed. 

In a commentary for the United Kingdom's Royal United Services Institute think-tank, senior research fellows Darya Dolzikova and Professor Justin Bronk highlighted the presence of layered ground-based air defences surrounding the facility. 

Iran possesses a wide range of air defences, including surface-to-air missile systems including the Russian S-300PMU-2 long range, Khordad 15 medium range and Russian Tor M-1 short range.

Having this array of systems near most key Iranian targets means some weapons may still be intercepted, particularly free-falling bombs, Ms Dolzikova and Prof Bronk wrote.

They cited how the Tor M-1 has repeatedly shot down a range of precision-guided munitions in the conflict in Ukraine.

The practical challenges involved in attacking a facility like Fordow render it infeasible to rely strictly on a military solution to Iran's nuclear threat, the experts wrote.

"Following any military strike on its nuclear sites, Iran not only has the requisite indigenous expertise but will also have increased incentive to rebuild, and to rebuild deeper and more hardened facilities."

But they still noted the "value of issuing credible military threats as part of a broader effort", particularly if Iran decides to rush towards a nuclear weapon, or begin uranium enrichment to 90 per cent.

"Making clear the willingness to carry out such attacks – but only as an option of last resort – could help exert the necessary pressure to convince Tehran to cooperate with expectations not to cross certain thresholds or to reverse course on particularly destabilising activity."