It's a gray late-September Sunday in San Francisco, a few weeks after the carnage at the World Trade Center, and my dog and I are walking along the beach near the Golden Gate Bridge. The huge structure seems graceful, spare and absolutely immovable. Yet with a brain steeped in movie special effects and, now, the all-too-real TV images of September 11, I can't help imagining the scene if a 767 were to rocket down out of the clouds, decapitating one of the bridge's towers or snapping the main suspension cables. I can see wires recoiling in slow motion, the main span sagging and shearing apart, cars and trucks and pedestrians plunging into the bay 67 meters below.

How many people are imagining similar horrors in their own cities, where the skylines remain unscathed? Ever since the seemingly invincible Twin Towers disappeared in a cloud of dust, Americans have been spooked about the danger to the national infrastructure. Naturally, we're wondering if anything can be done to harden skyscrapers against suicide attacks. But we're also reexamining our entire technological fabric-buildings and bridges and tunnels, stadiums and train stations and shopping malls, water supplies and the electrical grid, computer and telephone networks, roads and railways-and asking how it can be made more resistant to the predations of terrorists. "We have to start envisioning, and preparing for, worst-case scenarios," says Nancy Greene, president of the American Civil Defense Association, a Starke, FL-based nonprofit organization dedicated to preparing the population for natural and unnatural disasters. "People are finally starting to wake up, but unfortunately it takes this sort of action to make it happen."