Israel strikes Iran's nuclear facilities, but an expert cautions that the heavily protected Fordow site remains a dangerous threat that only U.S. military capabilities can neutralize.

Iran has two known underground nuclear enrichment sites, the one Israel attacked on the first day of its assault at Natanz, and another at Fordo. Both sit south of Tehran in central Iran, but Fordo has long been the Iranian nuclear facility of most concern to international monitors and experts.