Iran’s most heavily fortified nuclear site is at Fordo, a hilly area south of Tehran and about 15 miles away from Qum, a city where more than a million people live.

It was an unprecedented attack years in the making, with some last-minute misdirection meant to give the operation a powerful element of surprise.

U.S. strikes on Iran's nuclear sites prompt retaliation, raising questions about Tehran's nuclear program and military response.