• A silent, invisible electromagnetic pulse will instantly disable many computers, cars, and other electronic systems for miles.
• A blinding flash of light will bathe the area, burning the retinas of all looking directly into it. (Permanent blindness for some; temporary for others.)
• A crushing heat and shock wave, accompanied by a fierce wind, will knock down many buildings within a half mile. Beyond that immediate radius, most buildings will stay standing, but people and glass will get tossed about for many miles.

Soon fires will engulf thousands of buildings, and a large, deadly plume of radioactive dust will be carried in one direction or another by prevailing winds.

So, what should you do? For all survivors within 20 miles, the immediate task will be to stay away from fires and avoid the fallout for at least a couple of days. (The vast majority of radioactivity fades away that quickly.) The only two methods of avoiding fallout would be:

A) to take shelter until the radiation danger fades, or

B) if you have time, evacuate the area, heading in a perpendicular direction to the fallout wind.

In either case, it would be a very good idea for everyone in the exposed area to take potassium iodide pills, a relatively harmless substance that prevents your thyroid from soaking up radio-iodine and thus lowers the risk of future thyroid cancer. (Appropriate doses here. Good place to buy the pills here.) It would also be extremely useful to have a key-chain radiation monitor—the one that currently seems to be most effective is here.

Whether to stay put or run away is the subject of some controversy. In all likelihood, many survivors of the blast would quickly find themselves in an eerie simulation of every political satirist's favorite film, Duck and Cover.

What in the 1950s came across as a laughably reassuring response to an overwhelming threat turns out to be surprisingly coherent practical advice for the urban 21^st century. Because you are unlikely to be able to outrun the radioactive fallout, the best option in any city would most likely be to immediately find refuge under a thick physical barrier and to remain there for at least a few days. That barrier is your best defense against tiny particles and penetrating rays. Basements are best, followed by interior rooms with no windows. If you are in a tall tower, it's probably best to be on a midlevel floor, in a room close to the center of the building with no windows. You will need to stay there for several days at least, so your temporary shelter should be prestocked with food, water, radio, flashlights, and a makeshift toilet. Ideally, radio messages would begin soon after the explosion to instruct people about the nature and direction of the fallout and whether and how to evacuate. (Those interested in more thorough preparation can go here.)

"This would not be the end of the world," nuclear expert Charles Ferguson emphasized to me as we talked through the sequence of post-atomic events. "We can deal with this kind of horrific attack, and a little preparation can go a long way to increasing your chances of survival." It's a shocking, unnerving reality that one can rationally prepare for a nuclear blast. But all it really takes is a trip to the grocery store, a few clicks on the Internet, and short conversations with your boss and your wife.

I know that most of you would sooner shop for your own casket than stock up on post-nuclear groceries. The great paradox of surviving nuclear terrorism is that probably the most excruciating part is confronting it emotionally, tearing your psyche away from the much more comfortable (and widely assumed) scenario of annihilation. Ask any New Yorker about a nuclear attack and the first thing you'll hear is, "Why dwell on it? I'll be dead." No one wants to hear the muckier truth of likely survival. If confronted, people jerk back with the response, "I'd kill myself." But you wouldn't. A few survivors might, but that's just not what humans do in the face of catastrophe. Ask Elie Wiesel or Viktor Frankl. We've got this annoying survival instinct. You would want to live. You would want to help your family. You would want to help others. You would want to rebuild your life.

Why not face that reality now, in advance? Yes, it's uncomfortable. But an hour or two of preparation might mean the difference between complete misery and relative safety. No one is suggesting a life-altering obsession—Lord knows I'm looking forward to thinking about something else—only that you spend about as much time preparing for this awful unlikelihood as you already have for other awful unlikelihoods. None of us expect to get cancer or watch our house burn to cinders, but we buy health, life, and home insurance just in case. We prepare for the worst and then forget about it. Why not apply that same principle to acts of God or Bin Laden?

Maybe you won't, but I will. Someone has to start the trend. To survey my survival gear options, I paid a visit to Safer America, a disaster-preparedness supply shop on East 54^th Street in Manhattan.

General manager Jonathan Elkoubi was waiting inside, ready to show me how, since 9/11, he has helped families and corporations prepare for the next 9/11. His showroom is a survivalist's paradise, stocked with everything from smoke hoods and particle masks to earthquake alarms and skyscraper escape parachutes. ("I'm not going to name any names," he said, "but you'd be shocked by the CEOs who buy these parachutes for their own personal use and buy nothing for anyone else even on their own floor.")

At the end of the full tour, we came to the pièces de résistance: the new, lightweight Demron™ torso vest and full-body radiation suit. For $688 and $1,200, respectively, not only will you effortlessly beat back most ionizing rays—you'll look damned good doing it.

Maybe that's how we turn urbanites around on nuclear preparedness. We make it chic.

Click here for the essential survival shopping list. Next: How to survive an earthquake