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.

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