The whistle of incoming fire is distinctive. Once heard, it's not forgotten. But even those who know the sound have only a second or two to process the warning before the explosion rocks the ground beneath them.

"Get into the bunker! Get into the bunker!"

Someone is shouting. Actually, a lot of people are shouting. This isn't a drill! Diving to the floor is the first thought, but it's the wrong one.

Get out. Jump from the trailer. Run. The bunker is that way. There is an edge of barely suppressed panic in the voices of the Marines. Some have experienced this before; most have not. Most are 18 or 19, just in grade school during the Scud barrages of the Persian Gulf War in 1991.

The older sergeants keep their heads and make sure the young ones set out for the shelter.

There is still no siren. But the roar of the explosion was unmistakable. Bodies stumble over each other, sprinting to the bunker. A look back is a mistake. A large plume of black smoke and dust rises above the camp, just a little ways away. Is it smoke? Or maybe gas? Where is that gas mask? Where's the damn mask? Wait, it's in the little green satchel attached to the hip, right where it's supposed to be.

Fumbling for the mask. Where are those straps? Bodies pushing, shoving into the bunker. Everyone trying to cram inside. Impossible to pull the mask on. Just press it against the face until things calm down. Push it tight, have to get a seal, no air from the outside can get in. Don't worry about the straps. Just hold it tight.

More bodies trying to find cover. The bunker is a concrete hutch, maybe four feet tall and 25 feet long. Sandbags surround the entrance and the sides. Already inside Scud Bunker B-3 are perhaps 40 Marines, helmets on, gas masks on, rifles stuck between their legs as they sit on the dirt or on each other. A few try standing, hunched halfway over.

"Move in! Move in!"

It's hard to breathe. Everyone's huffing, gasping for air. It's impossible to get a breath. Is it gas? No, no, everyone's just winded. Take a breath. Settle down. Hearts are racing. Still hard to breathe. The mask is suffocating. The only way to get a deep breath is to take it off. Can't do that. Is there another explosion? What's happened with that plume? Is it coming this way? This is not a drill.

"Calm down! Calm down!"

The Marines begin to calm down, just a bit. They can only see each other's eyes, warped by the bug-eyed lenses of the gas masks. There is fear, yes, and adrenaline and uncertainty and most of all shock.

The missile attack Thursday was Iraq's first shot of the war. And it had been at them.

The rule in the morning had been that everyone would start to wear flak jackets and helmets, but the rule had been rescinded just a few minutes before the Iraqi missile exploded in a fireball near the camp.

The shouting and the confusion have begun to die down. Sobbing emerges from the other side of the bunker. A couple of the base's Arab employees have fled into the bunker, too, but they have no gas masks. One of them is heaving with tears.

"Someone comfort him," a major orders.

A Marine stands up, climbs over his buddies to reach the worker. "You're all right," he is saying, putting his arm around him. "It's going to be okay." It's not clear whether the words register, but the gesture does. The Marine points to the thick concrete wall to signal that it would protect him. The man regains control and settles down. It's not at all clear whether he is, in fact, going to be okay.

A few minutes after the explosion, the early-warning siren finally goes off. A voice echoes around the base. "Attention, Camp Commando. Attention Camp Commando." It is known as "the Big Giant Voice" even on official military documents and notices. But this time it's hard to hear. What's he saying? Something about getting into the bunkers.

Ten minutes pass, fifteen. No more explosions. Big Giant Voice says something about NBC teams -- nuclear, biological and chemical -- confirming there was no gas. Marines in the bunker slump back in relief, some of the tension draining out of their shoulders.

Humor returns. Grim jokes about the accuracy of the Patriot antimissile system.

A master sergeant makes a crack about being on the phone when the missile hit. "I was just asking Centcom for some help. You'd think they could've just said no." A few minutes later he alludes to the instructions drummed into troops during drills -- that early-warning sirens indicate incoming missiles. "I'm supposed to get a minute."

The dozens of Marines lumped on top of each other in the cramped bunker begin sharing personal stories and asking about one other's wives.

Eventually, a captain pulls out a disposable camera. "Group picture," someone calls out. The flash goes off.

The sound of Cobra attack helicopters buzzing overhead offers some reassurance. After an hour, Big Giant Voice weighs in again. "All clear. All clear." The masks come off.

The impact site quickly becomes a tourist attraction. The missile left a 50-foot-wide blast circle but only a two-foot crater before it skipped and scarred another part of the dirt. The Kuwaiti police team drives up and looks at it for three minutes, then leaves.

Back to work, but not long afterward, the siren suddenly goes off. A scramble to the bunker again. Mad shoving, pushing. Must get in. Not enough room. Make way!

"He's trying to get his licks in, ain't he?" a Marine grouses.

Sixteen times over 27 hours Marines are sent racing to the bunkers or their masks. Sixteen times the hearts race and the adrenaline runs and it never quite becomes routine.

Deep into the night, no one bothers to banter anymore. The cameras have been put away.

The only sound in the darkened bunker is the heavy breathing in the masks. Except someone has a phone, someone is being told of possible trouble nearby.

"North of Commando? We don't have anyone there, do we?"

Pause.

"Are they going to shoot it?"

Long pause.

Finally someone says, "I think I liked Afghanistan better."

More silence. Big Giant Voice gives the all-clear and Marines start to get out of the bunker.

"Might as well stay here," sighs one. "They'll call another one in a few minutes."

Outside, the masks come off. Then the siren comes back on. She was right. Get to the bunkers. This is not a drill.