For days, Israel's military has been bombing Iran, its main target:  Iran'snuclear facilities.

Israel is convinced that Iran is trying to build a nuclear weapon, an accusation that Iranian leadership has repeatedly rejected.

At the same time, over the past several decades, Iran has constructed numerous nuclear facilities all across the country.

Several of these are thought to house large underground bunkers where research that exceeds any civilian applications could be conducted in secret.

Heavy damage at Natanz and Isfahan

Until the attack, the Natanz Nuclear Facility in central Iran had been conducting large-scale uranium enrichment of up to 60% according to the International Atomic Energy Agency (IAEA). Between 3%-5% enrichment is required to run a nuclear power plant and 90% to build a nuclear bomb.  

IAEA Director General Rafael Grossi told the BBC that the above-ground centrifuges needed for such enrichment have been almost entirely destroyed following Israeli strikes at Natanz.

It is unclear whether subterranean portions of the facility have also been destroyed, but Israeli attacks also caused massive power outages that may have caused significant damage.

Grossi said it was possible that "dangerous radiation contamination" had occurred inside the facility, but that none had been detected outside.

At least four buildings at the Isfahan Nuclear Technology Center (INTC) have also sustained damage. One, Isfahan's Uranium Conversion Facility, was engaged in processing so-called yellowcake into uranium oxide then uranium tetrafluoride and uranium hexafluoride, key steps for further uranium enrichment.

Secret complex: The Fordo Fuel Enrichment Plant

Beyond Natanz, Iran has another important enrichment facility, the Fordo Fuel Enrichment Plant, south of the capital Tehran.

Situated on a former military base near the city of Ghom, Iran's leaders secretly installed the Fordo facility in the early 2000s. Israeli attacks are said to have targeted the site, though there have been no reports of serious damage so far.

That may be because a large part of the Fordo complex lies deep underground. To protect the site from possible attacks or sabotage and keep it out of view of IAEA inspectors, Iran had a system of 60-to-90-meter-deep tunnels (197 and 295 feet) drilled into the mountains.

International intelligence services first made the existence of the underground site public in 2009.

In 2012, the IAEA announced that scientists at Fordo had begun enriching uranium up to 20% "for medical purposes."

It is thought that a total of about 3,000 enrichment centrifuges have been installed at the underground site since then. Although Fordo is a smaller complex than Natanz, it is reportedly capable of producing purer grades of uranium, making it militarily far more significant.

Was Iran about to build a nuclear bomb?  

No outsiders know exactly what goes on at Fordo. Although the IAEA theoretically conducts inspections there, Iran has increasingly limited access for international inspectors and even removed monitoring cameras after the collapse of the JCPOA (Joint Comprehensive Plan of Action), or the 2015 Iran nuclear deal, which US President Donald Trump withdrew his country from during his first term.

In late May, the IAEA accused Iran of increasing its production of 60% enriched uranium, saying the country had amassed some 400 kilograms (882 pounds) of the stuff.

Weapons-grade enrichment could progress comparatively quickly at this point. In the days leading up to Israel's attack on Friday 13 June, the Institute for Science and International Security, a US think tank, published a report warning that Iran's known Fordo stockpile would allow the production of 233 kilos of weapons-grade uranium — enough to build several nuclear warheads — in just three week's time.

A tough target to hit

Its potential uranium-enrichment capabilities make Fordo a clear target for future Israeli attacks.

"The entire operation … really has to be completed with the elimination of Fordo," Israel's Ambassador to the US, Yechiel Leiter, told Fox News.

But destroying a facility buried deep beneath a mountain is especially difficult.

Military analyst Cedric Leighton told broadcaster CNN that Iran had engineered an especially hard concrete to protect the complex from air attacks.

Israel possesses bunker-busting weaponry but would still require several precise attacks to pierce the facility's protective shell.

The only bunker-busting bomb in the West big enough to achieve that task is owned by the United States. The precision-guided GBU-57 A/B MOP (Massive Ordnance Penetrator) weighs some 14 tons and was developed to reach targets located deep underground.

But the bomb is too large and too heavy for the Israeli air force to deliver. To do so would require a US B-2 or B-52 bomber. Whether the US will allow itself to be drawn into a direct conflict between Israel and Iran in the name of ending Iran's nuclear program remains an open question. 

This article was originally published in German and was translated by Jon Shelton.