The Dimona complex in the Negev desert is famous as the heavily guarded heart of Israel's never-acknowledged nuclear arms program. Over the past two years ... Dimona has taken on a new, equally secret role - as a critical testing ground in a joint American and Israeli effort to undermine Iran's efforts to make a bomb of its own. Behind Dimona's barbed wire ... Israel has spun nuclear centrifuges virtually identical to Iran's at Natanz, where Iranian scientists are struggling to enrich uranium. Dimona tested the effectiveness of the Stuxnet computer worm, a destructive program that appears to have wiped out roughly a fifth of Iran's nuclear centrifuges and helped delay, though not destroy, Tehran's ability to make its first nuclear arms. The operations [at Dimona], as well as related efforts in the United States, are among the newest and strongest clues suggesting that the virus was designed as an American-Israeli project to sabotage the Iranian program. In recent days, the retiring chief of Israel's Mossad intelligence agency, Meir Dagan ... told the Israeli Knesset in recent days that Iran had run into technological difficulties that could delay a bomb until 2015. That represented a sharp reversal from Israel's long-held argument that Iran was on the cusp of success. The biggest single factor in putting time on the nuclear clock appears to be Stuxnet, the most sophisticated cyberweapon ever deployed.