Five tunnel entrances, a white building and a heavily guarded security zone that stretches far over the mountains. That's all that can be seen on satellite images of the enrichment facility Fordo southwest of Tehran.
Deep inside the mountain, 60-percent enriched uranium is being enriched. The secret complex near the holy city of Qom has been eluding Israel and the USA since its existence was revealed in 2009. According to Iran, the facility was built as a result of Israel's repeated threats of military attacks.
Since Israel launched its Iran attacks on Friday, no damage to Fordo has been reported, according to the UN's atomic energy agency IAEA on Tuesday.
You have to blow up the mountain to reach it, says military analyst Michael Clarke to Sky News.
60 meters down
What little is known about Fordo comes from Iranian documents stolen by the Israeli intelligence service: that the enrichment halls are located far underground, possibly as deep as 90 meters.
That's too deep for any of the bombs Israel possesses. The only weapon that is considered to have the capacity to possibly hit Fordo is the American bunker bomb GBU-57 MOP (Massive Ordnance Penetrator).
We are the only ones who have the ability to do it, but that doesn't mean I'm going to do it, said President Trump on Wednesday about the possibility of destroying the facility.
The bomb is believed to have been developed specifically to be able to strike underground facilities in Iran and North Korea. GBU-57 weighs 13.6 tons - more than any other conventional bomb - and can drill through over 60 meters of reinforced concrete before it detonates.
Requires multiple bombs
The USA is also the only country in the world with the type of aircraft required to transport and drop the bomb: so-called B-2s, which can carry two GBU-57s each. The total of 19 American bomber planes are stationed at Whiteman Air Force Base in Missouri, USA - 30 hours of flying time from Iran, according to AP. The planes must be refueled several times to cover the distance, but this can be done in the air.
It will not be enough with a single bunker buster to take out Fordo, noted the British Royal United Services Institute (Rusi) in a report in March.
"Even GBU-57/B would likely require multiple hits at the same point to have a good chance of penetrating the facility," they wrote.
GBU-57 has, according to public information, never been used in combat, reports The Washington Post.
The question is also what would happen to the highly enriched uranium in the event of an attack on Fordo. There is a risk that it will be released into the air, writes AP.
Mia Holmberg Karlsson/TT
Facts: GBU-57 MOP
TT
The so-called "bunker buster" GBU-57 MOP (Massive Ordnance Penetrator) is a bomb that has been part of the USA's air force arsenal since 2011.
The bomb is 6.2 meters long and weighs 13.6 tons. It is developed to be able to destroy bunkers and tunnels located deep in the ground.
According to the US Air Force, GBU-57 can penetrate up to 61 meters of reinforced concrete before it detonates.
Over 80 percent of the bomb's weight consists of metal. Due to its weight and size, it can only be dropped by the American B-2 Spirit bomber planes. After that, it is guided to its target with GPS.
