The computer snafu that sent the Dow Jones industrial average plummeting 178 points in one minute on Feb. 27 was only the latest reminder of how dependent the U.S. public has become on computers -- and not just the laptops and handhelds many people carry between home and work.

Bigger swaths of most people's everyday lives are dependent on the smooth functioning of sophisticated -- but often vulnerable -- computer systems.

Consider the power grid's computers that keep power flowing in 15 states and one Canadian province. More than 50 million people were left without electricity on Aug. 14, 2003, thanks in part to a software bug. Consider the 13 Root Domain Name servers, two of which are operated by VeriSign that each day receive 25 billion requests for Web addresses.

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The system is a frequent target of computer malcontents, and a Feb. 6 attack caused three of the servers to drop 90% of their queries during a 12-hour period.

Or imagine life without Google, the Web search giant that accounted for 3.3 billion U.S. Web searches, or almost half the total, in January and is now providing e-mail services for tens of millions. The company's systems were hit by a slowdown caused by an outbreak of the MyDoom virus on July 26, 2004.

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And if it weren't already clear how critically modern civilization relies on the information stored in and processed by computers, consider this: A recent IDC study pegged the amount of data produced last year by the "digital universe" -- that is, all the world's computers, digital cameras, and other devices -- at 161 billion gigabytes, or 161 exabytes, which would be some 3 million times more information than in all the books ever published in all of human history. By 2010, the study forecasts, society will have generated nearly six times more digital information -- some 988 exabytes, or nearly a zettabyte.

Society's asking a lot of its computers. The attached slide show examines these and a host of other systems that keep the power flowing, airplanes flying, and the Internet buzzing. It focuses on everyday existence -- leaving aside, for instance, the many large supercomputers involved in such tasks as simulated testing of nuclear weapons -- and offers a glimpse of who's let down when the systems fail.

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