Iran's Natanz nuclear site has been a target of Israel's airstrikes/SCREENGRABIsrael has been targeting Iran's nuclear infrastructure, with a key focus on its underground uranium enrichment plants.
Its Natanz facility, in the centre of the country, has sustained severe damage, according to the International Atomic Energy Agency (IAEA).
Another site, Fordo, is buried deep in a mountain. Reaching this underground installation would require more powerful "bunker-busting" bombs, which only the US has.
So what would be the risks of bombing Fordo?
The IAEA has described the attacks on Iran's nuclear facilities as "deeply concerning."
On Monday, its Director General Rafael Grossi said that military escalation "increases the chance of a radiological release with serious consequences for people and the environment."
Uranium enrichment sites are used to build up supplies of a particular type—or isotope—of uranium.
"When you dig uranium out of the ground, it comes in two forms: 99.3% is uranium-238, and 0.7%, or about one atom in 150, is uranium-235, and this is what you need to work in your nuclear reactor," explains Professor Paddy Regan from the University of Surrey and the UK National Physical Laboratory.
Burst of energy
The process of nuclear enrichment basically means increasing the amount of uranium-235.
This is done by taking uranium in its gas form and spinning it in machines called centrifuges, Prof Regan said.
And because uranium-238 is heavier than the required uranium-235, the two separate out as they spin. This is repeated again and again to boost the enrichment.
Nuclear power stations typically need about 3-5% of this enriched uranium to generate a controlled nuclear reaction that releases energy.
But when the aim is to make a nuclear weapon, a much higher proportion of uranium-235 is needed - about 90%.
Essentially, the more enriched the uranium, the bigger the burst of energy when all those atoms split.
The IAEA said that Iran's uranium had reached about 60% enrichment—so well on its way to being concentrated enough for a nuclear weapon.
But firing a rocket into properly stored stockpiles of enriched uranium would not pose a "nuclear incident" on the same scale as disasters that occurred at nuclear power plants in Fukushima or Chernobyl.
"Highly enriched uranium is about three times more radioactive than non-enriched uranium. But in fact, on the scale of things, neither of them are particularly densely radioactive. It wouldn't cause a major environmental contamination problem," explains Prof Jim Smith, from the University of Portsmouth, who has studied the aftermath of the Chernobyl disaster.
"We're more concerned about what are called the fission products—the things that uranium splits up to when it's in a reactor or in a bomb—things like radioactive caesium, radioactive strontium, radioactive iodine. They are more of an environmental contamination issue."
But because no nuclear reaction is taking place at the enrichment sites - and a blast from a bomb would not trigger one - these dangerous radioactive "fission products" would not be present, he said.
Instead, the uranium could be dispersed locally by a blast.
