Usually, the incoming mortar is thrwarted with the eyes in the sky and the guys with long hair and thick necks and sharp knives. But when the wind thickens the air with dust, and surveillence becomes difficult, the klaxon will sound and send everyone to the ground. Sometimes defense missles shoot the mortar round out of the sky, sometimes it is a dud, and occasionally, it lands somewhere on this vast base stirring up the air, and shaking the ground.
Incoming fire of real consequence may not be common, but the sirens and announcements are. The base is well fortified and protected, but the enemy fervent and tireless. I have read (and this is common knowledge, not OPSEC) that al-Qaeda in Iraq has become more clever, and no longer places mortars in the fields around us themselves. Instead, they pay impoverished farmers to place "packages" in their fields, or, they have been known to place a mortar round frozen in a block of ice in the apeture of a launcher, whereas it may launch hours later after melting away the ice in the hot sun, and subsequently falling into its launching barrel. These tricks have low specificity, and limited payload, but are very difficult to prevent, thus, they harass us, and require vigilence to monitor and contain. More importantly, they generate fear and we expend more resources to account for them - and this is the terrorist victory difficult for us to deny, at home or abroad.
Of the many rounds which never detonate, EOD teams are quick to disassemble those warheads, but, in place of the klaxon, the loudspeakers issue instructions and caution - another auditory stimulus unique to a war zone. "THERE HAS BEEN AN INDIRECT FIRE ATTACK..." The klaxon and the intercom, like cordite and decay for the olfactory, are the indeliable sounds of Iraq; so askew from home, so poignantly descriptive of war.