For 40 years, we've been playing a game with terrorists. It's called "Hide the Bomb." We installed metal detectors in airports, so they devised nonmetallic explosives. We scanned bodies, so they hid a bomb in a shoe. We put everybody's shoes through scanners, so they hid bombs in underwear. We put people through scanners that showed the exact contours of their crotches and breasts. Citizens erupted in outrage. They said we had gone too far.

Actually, we didn't go far enough. Terrorists are taking the next logical step. They're trying to hide bombs inside their bodies.

Last year, after the attempted underwear bombing  in Detroit, British intelligence reported Internet chatter among jihadists about the possibility of surgically implanting explosives. By year's end, the SITE Intelligence Group had captured a detailed online exchange among al-Qaida followers. "What is your opinion about surgeries through which I can implant the bomb ... inside the operative's body?" one writer asked. Others advised him to stitch plastic or liquid explosives into the martyr's abdominal cavity.

Now the U.S. Department of Homeland Security is issuing an alert to foreign governments. An excerpt posted in the Daily Mirror  says: "Our Government has information indicating doctors have offered to help extremists surgically implant explosive devices in humans and animals for terrorist attacks." Another excerpt, quoted by Reuters  and ABC News, says DHS "has identified a potential threat from terrorists who may surgically implant explosives or explosive components in humans to conduct terrorist attacks." CBS News  reports that according to its intelligence sources, al-Qaida in the Arabian Peninsula, the outfit that plotted the underwear bombing, has shown "an interest in recruiting a surgeon to implant explosives in the body of a suicide bomber to circumvent airport detection equipment."

How would such a bomb work? Pretty much the way the jihadists proposed online, according to experts. Seal a plastic explosive such as PETN  in a packet, cut open your volunteer, insert the packet, stitch up the incision, and let it heal. In a man, the packet could go into the buttocks or abdomen. In a woman, it could be a breast implant. Give the bomber a syringe to inject TATP, which will detonate the bomb. Al-Qaida in the Arabian Peninsula has already used the PETN-TATP formula in the underwear bombing and a similar operation in Somalia. The Daily Mirror, apparently paraphrasing a memo  from the U.S. Transportation Security Administration, says implanted bombers "would carry a letter from a doctor claiming they have had surgery and need to carry a needle and syringe for medical reasons. It would be sufficient to clear security during check-in for a flight." The simplest medical excuse would be diabetes.

Airport scanners wouldn't catch this plot. They're superficial. The most advanced ones in use today can show your scrotum or nipples  in excruciating detail, but they can't see a bomb inside a terrorist. Several months ago, when TSA administrator John Pistole was asked whether "current technology" could detect an explosive "in a body cavity," he said no. "If they do a body cavity bomb, we're not going to detect that," he told USA Today. "We can't eliminate that risk." Yesterday, the vice president of Rapiscan Systems, which makes the backscatter machines used in U.S. airports, agreed. The machines, he explained, are "designed to detect threats on the body, not in the body."

Until now, the government has been betting that terrorists couldn't take down a plane with an internal bomb. "Every piece of information I have says the terrorists have not been successful in using ... a body cavity," Pistole told reporters late last year. (He thinks the 2009 Saudi assassination attempt originally ascribed to a rectal bomb  was actually a crotch bomb. It's hard to tell, since the bomber was blown to smithereens.) Detonating a bomb inside your rectum isn't easy. Surgery is much more complicated. Recovery would be difficult, and the bomber's body would smother the blast. As one jihadist noted in an online exchange, the bomb "must be planted near the surface of the body, because the human body absorbs shocks."

That gives Pistole hope that external screening would catch the bomber. "You have to have some external device" to detonate a cavity bomb, he argued last year, "and that's what the advanced imaging technology machine will pick up: any anomaly outside of the body." Or maybe swabs would find explosive residue on the bomber's clothes. Or the implant might cause telltale behavior. According to CBS News, security officers are being advised to look for "someone who may be acting unusual possibly due to an inserted device, which may be causing discomfort or pain."

But if there's no residue or behavioral giveaway, and if the detonator is under the skin, we're screwed. Eight months ago, Francine Kiefer of the Christian Science Monitor asked Pistole whether a terrorist could trigger a cavity bomb with a cell phone, as insurgents often do with IEDs. In response, she reported, Pistole "held out his arms and shrugged."

Now that the U.S. government is sounding international alarms about implanted explosives, the next step in "Hide the Bomb" is obvious: airport scanners that can see not just through your clothes, but through your whole body. Morpho, a global security firm, is working on a radio-wave device to detect what it calls "bombs in bodies." Nesch, an imaging company, advertises low-dose X-ray technology that can detect explosives "hidden both inside & outside of the human body." Valley Forge Composite Technologies, which makes screening devices for bombs and weapons, is marketing a radiographic imaging system that can "see through individuals" and spot "non-metallic items including sheet explosives hidden in clothing or in body cavities."

All three companies are telling investors, clients, and the government the same thing: In a world of underwear bombs, cavity bombs, and implanted bombs, see-through scanners are the wave of the future. And they're probably right, until terrorists figure out how to beat the system again.

Like Slate on Facebook. Follow us on Twitter. Human Nature's latest short takes on the news, via Twitter: