At half-past noon on Jan. 9, cable TV contractors sinking a half-mile of cable near Interstate 10 in rural Arizona pulled up something unexpected in the bucket of their backhoe: an unmarked fiber-optic cable. "It started pulling the fiber out of the pipe," says Scott Johansson, project manager for JK Communications and Construction. "Obviously, we said, 'Oop, we've hit something.'"

As the fiber came spooling out of the desert soil like a fishing line, long-distance service for millions of Sprint PCS and Nextel wireless customers west of the Rockies blinked off. Transcontinental internet traffic routed over Sprint slowed to a crawl, and some corporations that relied on the carrier to link office networks found themselves electronically isolated.

In the end, a hole dug out of a dirt road outside a town called Buckeye triggered a three-and-a-half hour outage with national impact. It wasn't even a very deep hole. "We ran into their line right away," says Johansson.

Experts say last week's Sprint outage is a reminder that with all the attention paid to computer viruses and the latest Windows security holes, the most vulnerable threads in America's critical infrastructures lie literally beneath our feet.

"No one wants something like this to happen," says Sprint spokesman John Taylor. "The fact is we are absolutely focused on restoring service to our customers ... and in this case we did so in record time."

A study issued last month by the Common Ground Alliance, or CGA -- an industry group comprised of utilities and construction companies -- calculated that there were more than 675,000 excavation accidents in 2004 in which underground cables or pipelines were damaged. And an October report from the Alliance for Telecommunications Industry Solutions found that cable dig-ups were the single most common cause of telecom outages over a 12-year period ending in 2004, with the number of incidents dropping in recent years but the severity and duration of the outages increasing.

In 2004, Department of Homeland Security officials became fearful that terrorists might start using accidental dig-ups as a road map for deliberate attacks, and convinced the FCC to begin locking up previously public data on outages. In a commission filing, DHS argued successfully that revealing the details of "even a single event may present a grave risk to the infrastructure."

"We see people talking about the digital Pearl Harbor from the worms and Trojans and viruses," says Howard Schmidt, former White House cybersecurity adviser. "But in all probability, there's more likelihood of what we call the 'backhoe attack' that would have more impact on a region then a Code Red, or anything we've seen so far."

Sprint claims it's still investigating who was at fault in Buckeye, but Johansson says that's a settled issue: Before his crew members disturbed so much as a pebble, they submitted their plans to Arizona's "call-before-you-dig" One Call center, then waited for each utility to mark off their buried facilities, if any. Contacted by Wired News, the center confirmed the call.

According to Johansson, Sprint responded by giving the contractors the all-clear. "We had a no-conflict ticket from them, indicating that they had no line there," he says.

Even that apparent gaffe wouldn't have been enough to cause an outage on its own. The Arizona fiber cut was on a transmission line that loops across the county in a solid ring -- a "self-healing" topology that guarantees a single break won't stop service, because traffic can always circle back in the other direction.

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