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 U.N. 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, U.N. 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...