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President Trump has threatened to strike Iran’s Pickaxe Mountain nuclear site during a speech at the Republican midterm convention in Dallas on Wednesday, citing activity at the facility.

Getting into it: Speaking to the crowd, Trump said, “We notice there’s a little activity at Pickaxe. I would advise Iran not to get cute because we will have to hit them very hard.” He has been threatening to strike the site since at least mid-July and told reporters last week that “we may hit Pickaxe very soon.”

Pickaxe Mountain, known in Farsi as Kuh-e Kolang Gaz La, is a tunnel complex buried in granite about a mile south of the Natanz enrichment facility. Iran broke ground there in 2020, announcing plans for an underground centrifuge assembly facility after Israel was suspected of sabotaging an above-ground one at Natanz. The International Atomic Energy Agency has pressed Iran for details on the site’s purpose and for inspector access, but Iran has never answered.

A new analysis of six years of satellite imagery by the Center for Strategic and International Studies (CSIS) found the roads around the site have never been busier than they are in 2026. “There is a clear surge in construction activity in 2026 that corresponds to the raising and hardening of the portal accesses, paving of the internal road network, reinforcement around access areas, and the flattening and removal of excavation spoil,” the think tank wrote. CSIS reads that as a sign Iran is likely moving past the digging phase and into building out the interior. The imagery shows the south and east entrances have been built up significantly since September 2025. CSIS also found the work continued at a rapid pace while the June 17 memorandum of understanding between Washington and Tehran was in effect, even though the deal required Iran to maintain the status quo of its nuclear program.

What is inside remains unknown. Israeli intelligence told the US last fall that Iran had moved uranium centrifuges and components into the site. CSIS says its imagery can neither confirm nor refute that, but notes the site still looks like it’s under construction, security hasn’t been beefed up, and only construction vehicles have been spotted, all signs the facility is “not yet capable of enrichment work.” The think tank lays out three possibilities: a centrifuge assembly plant, an enrichment facility that could push Iran’s roughly 440 kilograms of 60% enriched uranium to weapons grade, or a new home for weapons-related research relocated from the Isfahan facilities destroyed in June 2025.

Hitting it would be even harder. The biggest conventional bomb the US has for striking underground targets, the 30,000-pound GBU-57 Massive Ordnance Penetrator, can burrow about 200 feet into the ground before detonating. Pickaxe sits at least 328 feet below the surface, well beyond that range. At Fordow in June 2025, the Air Force dropped 12 penetrators using a double-tap technique, sending a second bomb into the crater made by the first, and succeeded partly because ventilation shafts channeled the blast underground.

This all comes as CNN recently revealed that the US is testing a next-generation penetrator bomb, with secret tests indicating it could dig through the same type of stone found at Pickaxe Mountain. It is unclear how far along that weapon is or whether it is ready to deploy.