US President Donald Trump has threatened to attack a site linked to Iran's nuclear program known as Mount Axe, a fortified facility buried deep underground near one of Iran's main nuclear sites.

Trump said in an interview on July 13: 'We will destroy Mount Axe. Tell the Iranians to prepare.' The threat reflects escalating tensions as Tehran and Washington exchange strikes in the Gulf, hampering efforts to end the conflict.

Here is what we know about Mount Axe:

Where is it located?

Mount Axe is located 220 kilometers south of Tehran and two kilometers from the Natanz nuclear complex.

The Natanz site, which houses two Iranian uranium enrichment plants, was bombed during the war launched by the United States and Israel on February 28, and during the 12-day war last year.

The Institute for Science and International Security, a US-based research center focused on preventing nuclear weapons proliferation, said the facility still under construction at Mount Axe was not targeted in either of those wars.

The mountain peak rises to about 1,600 meters above sea level.

There were two enrichment plants operating at Natanz, one above ground and one underground. The UN's International Atomic Energy Agency said the above-ground plant was destroyed. The other underground plant likely suffered at least severe damage.

What is the history of this site?

The site is linked to Iran's nuclear program, which has long strained relations between the West and Iran, which denies seeking an atomic bomb.

The institute said construction of the facility at Mount Axe began in 2020 following what Iranian authorities at the time said was an explosion caused by sabotage at the Natanz facility.

Iran said at the time that the sabotage at Natanz caused severe damage that could slow the development of advanced centrifuges for uranium enrichment.

In September of that year, then-head of the Atomic Energy Organization of Iran, Ali Akbar Salehi, said Iran had begun building 'a more modern, larger, and more comprehensive hall in the heart of the mountain near Natanz' to manufacture advanced centrifuges.

UN International Atomic Energy Agency Director General Rafael Grossi indicated in an interview with PBS Frontline in March that Iran had previously announced its intention to conduct nuclear activities at Mount Axe. He said: 'This was part of their very systematic intention to put their most sensitive facilities underground.'

What has Iran built there?

The institute, which has analyzed satellite images of the site, says it has two entrances that are presumed to lead to a single facility estimated to be at least 100 meters deep beneath the mountain.

In a report released on July 14, the institute said the physical defense measures consist mainly of a large security perimeter and extensive reinforcements of the tunnel entrances.

The institute's report noted that the two eastern tunnel entrances have been partially filled since the wars to block ground vehicle access, but not completely closed.

Sam Lair, a researcher at the Foreign Policy Research Institute, who also reviewed recent satellite images of the site, told Reuters that reinforcing the entrances would complicate 'targeting them with penetrating munitions such as bunker-buster bombs.'

Is the site operational?

Trump said in his remarks on July 13 that Washington is closely monitoring Mount Axe. He added: 'We don't see any activity there. They are not doing well with their nuclear situation. Every time we hear about it, we blow it up. So they don't like to talk about it. But we will probably strike Mount Axe relatively soon.'

The institute said in its report that its assessment 'indicates that the facility has not yet started operation, but construction is ongoing,' and it is unclear when it could begin operation based on satellite imagery alone.

What is the potential purpose of its construction?

The center stated: 'It is also unclear whether Iran still plans to build a large-scale assembly facility, given the destruction of Iran's centrifuge program, including Iran's ability to manufacture centrifuge components needed for an assembly plant.'

It continued: 'However, if Iran begins to rebuild its centrifuge manufacturing capability, it may plan to build a smaller centrifuge assembly facility at Mount Axe capable of serving a nuclear weapons program.'

How can the site be attacked?

Experts believe the deep-buried complex is beyond the reach of the most powerful bunker-buster bombs in the US arsenal.

The institute said the site 'would be more suitable for attack or sabotage by ground forces.'

It added: 'However, there may be vulnerabilities that could be exploited by deep-penetrating weapons via air strikes.'

Lair said: 'We can deduce that there are ongoing activities at Mount Axe that the Iranians wish to continue, but they are still worried enough about the possibility of an attack that they are taking steps to strengthen their defenses.'