A US think tank, the Institute for Science and International Security (ISIS), says it has identified an intact, partially underground bunker at Iran's Natanz nuclear complex that appears to have escaped American and Israeli airstrikes in 2025 and 2026. The finding matters because it suggests Iran may retain infrastructure capable of supporting large-scale uranium enrichment despite two rounds of military action aimed at crippling its nuclear programme, and raises questions about why the site was not targeted and whether Tehran could use it to resume weapons-related work undetected.

ISIS says the bunkered structure was built between 2010 and 2017, mainly from 2015 onwards, and was likely intended to store uranium hexafluoride to support enrichment at the Natanz Fuel Enrichment Plant, though no vehicle activity has been seen there since the June 2025 war. Iran nearly doubled its stockpile of 60%-enriched uranium to 440kg between December 2024 and June 2025, according to an earlier ISIS report, fuelling fears it was nearing weapons-grade capability. Iran is refusing to allow International Atomic Energy Agency inspections of bombed sites, a stance experts say breaches its non-proliferation obligations, while the IAEA notes it previously inspected the building regularly before mid-2025. Analysts including Jason Brodsky of United Against Nuclear Iran argue the site should be destroyed in any future strikes and that its existence highlights flaws in the earlier 2015 nuclear deal.