Al Qaeda's hidden weapon.

In the wake of the failed bombing attempt by Nigerian Al Qaeda operative Umar Farouk Abdulmutallab, airport security experts are wringing their hands over how to stop the next underwear bomber. X-ray machines don’t detect the type of explosive, known as PETN, that Abdulmutallab carried. Only a careful pat-down around Abdulmutallab’s crotch, where the explosive had been sewn into his undies, would have detected his deadly cargo. But Abdulmutallab’s al Qaeda handlers knew that pat-downs are rare and that social mores make highly intrusive, crotch-fondling searches almost unheard of. In the wake of the Abdulmutallab episode, however, standards will change. Pat downs will become more common—and more intrusive. We may not see the famous vision of the crazed dictator from Woody Allen’s Bananas—“Underwear shall be worn on the outside!”—but those searches by hand are likely to get a little more, shall we say, intimate.

Even a pat-down thorough enough to simulate foreplay, however, won’t protect us completely—not from a threat that sounds even more absurd than an underwear bomb and that is also more alarming: the butt bomb.

The concept is simple. Rather than sew explosives into his underwear, a terrorist might actually plant a bomb, which can weigh as little as a pound, inside his anal cavity. Like drug mules, would-be butt bombers could store the explosives inside a condom.

Sound crazy? Perhaps. Disgusting? Definitely. But security experts initially believed that a terrorist’s derriere nearly killed a top Saudi Arabian counterterrorism official last fall. Back in August, an Al Qaeda-connected militant named Abdullah Assiri offered to turn himself into Saudi authorities and enlist in a state-run terrorist rehabilitation program. Exhibiting a healthy skepticism, the Saudis reportedly subjected Assiri to two airport-style X-ray scans and other security checks. Finding no weapons or explosives on his body, security agents ushered Assiri into the palace of the counterterrorism chief, Prince Muhammad Bin Nayef, who is also the son of a likely heir to the Saudi throne.

Instead of surrendering, however, Assiri exploded. Nayef survived the blast, but the Saudis were bewildered by this incredible breach of their security. At first, they were convinced the explosive had been hidden in Assiri’s anal cavity—a scenario that other security experts didn’t discount. After further investigation, the Saudis concluded that Assiri didn’t have a butt bomb after all, but rather that he stashed the explosive in his underwear much like Abdulmutallab. (The device may have been detonated by a text message sent to Assiri’s cell phone; exactly how the phone triggered the bomb is unclear. Like Abdulmutallab, incidentally, Assiri appears to have gotten his assignment and materials in Yemen.)