At the oldest and largest uranium enrichment site, in Natanz, about 260 kilometers (160 miles) southeast of Tehran, an underground hall can hold up to 50,000 centrifuges. Iran says they only are used for energy, science or medicine. But they also could be used to churn out the fissile core of a nuclear warhead.
A second site, Fordo, was built into a mountainside south of Tehran. Iran acknowledged its existence six years ago after the United States shared intelligence about it with allies. The hardened underground bunker is thought to be resistant, if not impervious, to air attack.