When the director general of the International Atomic Energy Agency (IAEA) asked Iran to explain what was happening deep underneath Pickaxe mountain, the reply was short and sharp: “It’s none of your business.”

Rafael Grossi’s question is more urgent now, after Iran’s key enrichment facilities at Fordow and Natanz were targeted by American B-2 stealth jets armed with 30,000lb bunker-busting bombs in an attack Donald Trump claimed “obliterated” Iran’s nuclear programme.

Before the strike, 16 lorries were seen queuing up outside Fordow, and an expert on Iran’s nuclear programme told The Telegraph the regime had moved much of its highly enriched uranium to a secret location before the US was able to bomb its facilities.

Tehran had hidden sites housing “hundreds if not thousands” of advanced centrifuges capable of producing weapons-grade uranium needed for a nuclear bomb, said Sima Shine, who has worked within the Israeli military establishment for 30 years.

Pickaxe mountain may be the perfect hiding place.

The sprawling site at Kūh-e Kolang Gaz Lā is 90 miles south of Fordow, and only minutes away from the Natanz nuclear facility in central Isfahan province.

The installation, which is still under construction, has been reinforced and quietly expanded in the past four years.

“Since it is obvious it is in a place where numerous and important activities related to the programme are taking place, we’re asking them, ‘What is this for?’ And they are telling us, ‘It’s none of your business,’” Mr Grossi said in April.

He said it “cannot be excluded” that the tunnels would store undeclared material.