When a force as terrible as Hurricane Katrina rearranges the world, we grasp for the less-than-cataclysmic things we can see to understand the thoroughly cataclysmic things we can't. A house falling off its foundation makes more sense, in a photograph, than a blank patch of concrete where a demolished house once stood. A boat on its side, in the middle of a street, suggests destruction in a way that a clot of timber that was once a boat does not.

For a while, on Monday, one of the more arresting images from New Orleans was the Hyatt Regency Hotel, its windows blasted out, curtains flapping in the wind, pink insulation exposed to the elements. Compared with other damage, whole neighborhoods flooded to the rooftops, a breached levee that was sluicing water into the already sodden city, the windows of the Hyatt are a minor loss. But for anyone who has spent too much time in hotels, who has lived in the strange bubble world of the business traveler, the windows of the Hyatt were a marker for a more fundamental vulnerability.

For $150 a night, the large name-brand hotel offers the illusion of safety, the assurance that you are not set down among foreign people, strange customs, incomprehensible languages. Hotels do not try to pretend that they can give the illusion of home, a familiarity dense with personal meanings. They offer instead a neutral familiarity, rooms that never change from continent to continent, the same soap, the same towels, the same cable channels. The art is comfortably anodyne, the food tasteless but digestible.

And the windows? When we enter the hotel room, that wall of glass seems so inviting, as if we might sit in the lone chair by the lone table and stare out at the world, safely held at bay by impregnable transparent panels. But we don't. The rubberized inner curtain is soon drawn, the room sunk into inky darkness or artificial light, and the world we've come to visit safely blotted out, no sights, no sounds, no smells.

As the storm subsided and camera crews made their way into the disaster zone, it was clear that a wall of windows in a luxury hotel was only one minor line of security breached by the storm. Everywhere the visible and unseen barriers that divide our world were being redefined, or ruptured. Men with axes broke through roofs to provide escape for those who had sought refuge in their attics. A levee that held Lake Pontchartrain at bay was broken. The hurricane had shifted the line that divides land and water decidedly in favor of the latter.

Invisible lines were crossed as well. Oil passed the $70-a-barrel mark. The president drew a line on the end of his vacation, deciding to return to Washington today to oversee disaster relief efforts. And looters crossed that tenuous but essential line that divides the civilized from the selfish.

The country itself had a new line in it, a line dividing those scathed by the storm and the rest of us, who watched it, with sadness, fascination and the usual guilt of voyeurs. There's been a convention in the theater world to think of the division between audience and spectacle as a fourth wall, a wall that the playwright tries to eliminate through the force of his drama. But as images poured forth of hungry, exhausted, terrified and soaked survivors, anyone with any sense was happy for any kind of barrier that kept this drama, this devastation at bay.

Disasters never really teach us anything new, just the same old lessons pounded home once again with renewed force. We are frail. Our world is a compromise made with forces that can, and will, scatter it from time to time. Our homes, which we often treat as trophies of our ambition or investments for our future, ultimately matter to us because they are the structured memory of our lives.

Disaster reveals human nature, or so says the cliche. So often it seems that the good and the bad in human nature cancel themselves out. Some people send aid while others loot. Some people go forward, others despair. Volunteers pour in, and so do the swindlers, the disaster opportunists who prey on anyone desperate for a new roof.

It is a strength and a failing of Americans that they are incapable of imagining their own world in ruins. We look at our fertile and familiar land and find it inconceivable that it might be devastated by war, plagued by locusts, riddled by famine. Disasters elsewhere, on other continents with different victims, seem the natural course of the world. Disasters here mess with the very nature of reality. It's an illusion of invulnerability and an astonishingly persistent one.

With hotels like the Hyatt, we have extended this illusory zone of security around the world. Katrina did far worse damage, by orders of magnitude, than some shattered glass at the Hyatt. But it was the image of a hotel -- a refuge against the world -- that first suggested the degree to which Katrina would bring a foreign world, of chaos and frailty, into the comfort zone of American security.

Rooms on the north side of the Hyatt Regency Hotel in New Orleans are exposed to the fury.