Iran's Fordo nuclear facility sits deep inside a mountain, with just one structure visible from space.
Western intelligence officials have worried about the site since 2008, when satellite images caught work at the site just outside of the Shiite holy city of Qom, some 120 kilometers (75 miles) southwest of Iran's capital, Tehran.
Israel and the U.S. worried that the size of the facility, its hardened position and its air defenses signaled that it could be used for military activities — such as potentially enriching uranium for a nuclear weapon.