Drownings are often difficult to spot; they are rarely the splashy, flailing events depicted on television. Most are near-silent episodes where the victim quickly sinks out of view.

On this particular day maybe the lifeguards weren't paying as close attention as they should have been. Certainly they believed the trim, athletic LeRoy was not a high-risk swimmer.

But on this evening LeRoy was practicing  apnea swimming—testing how far he could swim underwater on one breath—and at some point, without making any visible or audible disturbance on the water's surface, he blacked out. The guards failed to notice as he stopped swimming and descended to the bottom of the deep end of the pool. With his arms crossed over his head and his feet twitching, he was unconscious and drowning. It would take him as little as four minutes to die.

Although the human lifeguards watching the pool were oblivious, 12 large machine eyes deep underwater were watching the whole thing and taking notice. Just nine months earlier the center had installed a state-of-the-art electronic surveillance system called Poseidon, a network of cameras that feeds a computer programmed to use a set of complex mathematical algorithms to distinguish between normal and distressed swimming. Poseidon covers a pool's entire swimming area and can distinguish among blurry reflections, shadows, and actual swimmers. It can also tell when real swimmers are moving in a way they're not supposed to. When the computer detects a possible problem, it instantly activates a beeper to alert lifeguards and displays the exact incident location on a monitor. The rest is up to the humans above the water.

Sixteen seconds after Poseidon noticed the large, sinking lump that was Jean-François LeRoy, lifeguards had LeRoy out of the pool and were initiating CPR. He started breathing again. After one night in the local hospital, he was released with no permanent damage. Poseidon—and, more precisely, the handful of French mathematicians who devised it—had saved his life.

Machines like Poseidon will redefine how we live. Think of your life before the answering machine, the ATM, e-mail. Think of your grandparents' lives before the television and the airplane. Think of your great-grandparents' lives before the telephone. All told, the shift will be that substantial. Machines will recognize our faces and our fingerprints. They will watch out for swimmers in distress, for radioactivity- and germ-laden terrorists, for red-light runners and highway speeders, for diabetics and heart patients.

Imagine devices that monitor the breathing rhythms of infants in cribs, watch toddlers at day care, and track children as they go to and from school; that can keep an eye on our home supply of orange juice and let us know when the milk is sour. Machines might watch our calorie intake and burn-off, monitor air quality in our homes, and look out for mice and bugs.

Envision sensors as large as walls and as small as molecules in your bloodstream sending quiet signals to nearby computers, which will process and relay information to you, your doctor, your lawyer, your grocer, your building manager, your car mechanic, your local fire or police department. As time and technology march on, less and less will escape the attention of sophisticated machines. They'll have us covered.

Get the whole story in the pages of National Geographic magazine.