From the article: SCADA systems -- computer systems that control industrial processes -- are one of the ways a computer hack can directly affect the real world. Here, the fears multiply. It's not bad guys deleting your files, or getting your personal information and taking out credit cards in your name; it's bad guys spewing chemicals into the atmosphere and dumping raw sewage into waterways. It's Stuxnet: centrifuges spinning out of control and destroying themselves. Never mind how realistic the threat is, it's scarier."
What worries Bruce Schneier most is that industry leader Siemens is keeping its SCADA vulnerabilities secret
, at least in part due to pressure from the Department of Homeland Security