“Iran’s uranium enrichment facility at Natanz may have had one of the most secure computer systems in the world. The building housing the nuclear program’s equipment is underground, protected by a combination of concrete walls, earth and military guards.

And it was a ‘closed’ network, sealed off from the Internet and unsusceptible to vulnerabilities in the system’s Windows-based software.”

Source:DoD Looking to ‘Jump the Gap’ Into Adversaries’ Closed Networks | Defense News | defensenews.com