VIENNA - In an underground chamber near the Iranian city of Natanz, a network of surveillance cameras offers the outside world a rare glimpse into Iran's largest nuclear facility. The cameras were installed by UN inspectors to keep tabs on Iran's nuclear progress, but last year they recorded something unexpected: workers hauling away crate after crate of broken equipment.In a six-month period between late 2009 and last spring, UN officials watched in amazement as Iran dismantled more than 10 percent of the Natanz plant's 9,000 centrifuge machines used to enrich uranium. Then, just as remarkably, hundreds of new machines arrived at the plant to replace the ones that were lost.The story told by the video footage is a shorthand recounting of the most significant cyber-attack ever on a nuclear installation. Records of the International Atomic Energy Agency (IAEA), the UN nuclear watchdog, show Iran struggling to cope with a massive equipment failure just at the time its main uranium enrichment plant was under attack by a computer worm known as Stuxnet, according to Europe-based diplomats familiar with the records.But the IAEA's files also show a feverish - and apparently successful- effort by Iranian scientists to contain the damage and replace broken parts, even while constrained by international sanctions banning Iran from purchasing nuclear equipment. An IAEA report due for release this month is expected to show steady or even slightly elevated production rates at the Natanz enrichment plant over the past year.