To make matters worse, phone systems are rarely designed to allow more than 10 percent of the population to talk simultaneously, and far more people than that rush to the telephone in an emergency. In the New York City blackout of 2003, while most land lines continued to function, the cellphone circuits were overjammed.

Katrina posed even worse problems. As phone traffic surged, the water was destroying a vast area, including underground phone lines. Mobile-phone networks, too, were ruined, because they're routed through communication towers that crumpled like paper in Katrina's 140-mile-an-hour winds. As a final insult, Katrina knocked out the power grid in swaths of the Gulf Coast -- which was fatal for phone systems that require thousands of watts of juice. The surviving mobile-phone sites in New Orleans could run on diesel-generator backup, but with just one tank of gas each, they were capable of operating for only a few days. Even the mayor nearly lost contact with the outside world. After their satellite phones ran out of power, employees of the mayor's office broke into an Office Depot and lifted phones, routers and the store's own computer server.

WiFi meshes elegantly dodge our phone system's central problems. They're low-power and ultracheap -- and decentralized like the Internet itself, which was initially conceived to withstand a nuclear attack. You can use WiFi to build a do-it-yourself phone system that is highly resistant to disaster.

In Chicago, the Center for Neighborhood Technology, a nonprofit organization, hooked up dozens of households in the neighborhoods of North Lawndale and Pilsen with WiFi nodes that form a mesh. Each node can communicate with its neighbor a few hundred feet away; by cooperating in this fashion, they form an enormous bucket brigade, each passing the data signal along until everyone is sharing it. If one single household connects to the Internet, all the other households can instantly dip in. Best of all, the WiFi mesh can handle not only data but also phone calls -- via the magic of "voice over IP," an increasingly popular technique for transmitting conversation over the Internet. Should the local phone lines suddenly collapse, the residents of these neighborhoods can still make calls to one another using headsets attached to their computers. In essence, they are their own backup phone company.

Unlike a normal land-line or mobile phone system, a WiFi mesh has no single weak point. Knock out any single node in one of the Chicago neighborhoods -- destroy an entire house, for that matter -- and the mesh has enough redundancy to work around the missing link. The nodes are also durable; they're tiny shoe-box-size devices, which means they're far less likely to be wiped out by hurricanes than enormous mobile-phone-company antennas. "We've been running these little Apollo 13 disaster scenarios where a bunch of our nodes get taken out, and the whole system just reconfigures itself automatically," said Paul Smith, who helped build the Chicago networks.