But when the IAEA later reviewed footage from surveillance cameras installed outside the cascade rooms to monitor Iran's enrichment program, they were stunned as they counted the numbers. The workers had been replacing the units at an incredible rate -- later estimates would indicate between 1,000 and 2,000 centrifuges were swapped out over a few months. The question was, why? What the inspectors didn't know was that the answer they were seeking was hidden all around them, buried in the disk space and memory of Natanz's computers. Months earlier, in June 2009, someone had silently unleashed a sophisticated and destructive digital worm that had been slithering its way through computers in Iran with just one aim -- to sabotage the country's uranium enrichment program and prevent President Mahmoud Ahmadinejad from building a nuclear weapon.