As numerous eight-year-old boys had discovered, you could not climb the dog pod grid because the pods didn't have enough thrust to support your weight; your foot would just mash the first pod into the ground. It would try to work its way loose, but if it were stuck in mud or its turbines fouled, another pod would have to come out and replace it. For the same reason you could pluck any pod from its place and carry it away. When Hackworth had performed this stunt as a youth, he had discovered that the farther it got from its appointed place the hotter it became, all the while politely informing him, in clipped military diction, that he had best release it or fall victim to vaguely adumbrated consequences. But nowadays you could just steal one or two whenever you felt like it, and a new one would come out and replace it; once they figured out they were no longer part of the grid, the pods would self-scramble and become instant souvenirs.

This user-friendly approach did not imply that grid-tampering went ignored, or that such activities were approved of. You could walk through the grid whenever you chose by shoving a few pods out of the way– unless Royal Security had told the pods to electrocute you or blast you into chum. If so, they would politely warn you before doing it. Even when they were in a more passive mode, though, the aerostats were watching and listening, so that nothing got through the dog pod grid without becoming an instant media celebrity with hundreds of uniformed fans down in Royal Joint Forces Command.

Unless it was microscopic. Microscopic invaders were more of the threat nowadays. Just to name one example, there was Red Death, a.k.a. the Seven Minute Special, a tiny aerodynamic capsule that burst open after impact and released a thousand or so corpuscle-size bodies, known colloquially as cookie-cutters, into the victim's bloodstream. It took about seven minutes for all of the blood in a typical person's body to recirculate, so after this interval the cookie-cutters would be randomly distributed throughout the victim's organs and limbs.

A cookie-cutter was shaped like an aspirin tablet except that the top and bottom were domed more to withstand ambient pressure; for like most other nanotechnological devices a cookie-cutter was filled with vacuum. Inside were two centrifuges, rotating on the same axis but in opposite directions, preventing the unit from acting like a gyroscope. The device could be triggered in various ways; the most primitive were simple seven-minute time bombs.

Detonation dissolved the bonds holding the centrifuges together so that each of a thousand or so balhisticules suddenly flew outward. The enclosing shell shattered easily, and each ballisticule kicked up a shock wave, doing surprisingly little damage at first, tracing narrow linear disturbances and occasionally taking a chip out of a bone. But soon they slowed to near the speed of sound, where shock wave piled on top of shock wave to produce a sonic boom. Then all the damage happened at once. Depending on the initial speed of the centrifuge, this could happen at varying distances from the detonation point; most everything inside the radius was undamaged but everything near it was pulped; hence, "cookie-cutter."

The victim then made a loud noise like the crack of a whip, as a few fragments exited his or her flesh and dropped through the sound barrier in air. Startled witnesses would turn just in time to see the victim flushing bright pink. Bloodred crescents would suddenly appear all over the body; these marked the geometric intersection of detonation surfaces with skin and were a boon to forensic types, who could thereby identify the type of cookie-cutter by comparing the marks against a handy pocket reference card. The victim was just a big leaky sack of undifferentiated gore at this point and, of course, never survived.

Such inventions had spawned concern that people from Phyle A might surreptitiously introduce a few million lethal devices into the bodies of members of Phyle B, providing the technically sweetest possible twist on the trite, ancient dream of being able instantly to turn a whole society into gravy. A few inroads of that kind had been made, a few mass closed-casket funerals had been held, but not many. It was hard to control these devices. If a person ate or drank one, it might end up in their body, but it might just go into the food chain and get recycled into the body of someone you liked. But the big problem was the host's immune system, which caused enough of a histological fuss to tip off the intended victims.

What worked in the body could work elsewhere, which is why phyles had their own immune systems now. The impregnable-shield paradigm didn't work at the nano level; one needed to hack the mean free path. A well-defended clave was surrounded by an aerial buffer zone infested with immunocules– microscopic aerostats designed to seek and destroy invaders. In the case of Atlantis/Shanghai this zone was never shallower than twenty kilometers. The innermost ring was a greenbelt lying on both sides of the dog pod grid, and the outer ring was called the Leased Territories.

It was always foggy in the Leased Territories, because all of the immunocules in the air sensed as nuclei for the condensation of water vapor. If you stared carefully into the fog and focused on a point inches in front of your nose, you could see it sparkling, like so many microscopic searchlights, as the immunocules swept space with lidar beams. Lidar was like radar except that it used the smaller wavelengths that happened to be visible to the human eye. The sparkling of tiny lights was the evidence of microscopic dreadnoughts hunting each other implacably through the fog, like U-boats and destroyers in the black water of the North Atlantic.

Nell sees something peculiar;

Harv explains all.

One morning Nell looked out the window and saw the world had turned the color of pencil lead. Cars, velocipedes, quadrupeds, even power-skaters left towering black vortices in their wakes. Harv came back from being out all night. Nell screamed when she saw him because he was a charcoal wraith with two monstrous growths on his face. He peeled back a filter mask to reveal grayish-pink skin underneath. He showed her his white teeth and then took up coughing. He went about this methodically, conjuring tangles of spun phlegm from his deepest alveoli and projecting them into the toilet. Now and then he would stop just to breathe, and a faint whistling noise would come from his throat.

Harv did not explain himself but went about working with his things. He unscrewed the bulges on his mask and took out black things that kicked up little black dust storms when he tossed them onto the floor. He replaced them with a couple of white things that he took from a Nanobar wrapper, though by the time he was finished, the white things were covered with his black fingerprints, the ridges and whorls perfectly resolved. He held the Nanobar wrapper up to the light for a moment. "Early protocol," he rasped, and pitched it toward the wastebasket.

Then he held the mask up to Nell's face, guided the straps around her head, and tightened them down. Her long hair got caught in the buckles and pinched, but her objections were muffled by the mask. It took a little effort to breathe now. The mask pressed against her face when she inhaled and whooshed when she exhaled.

"Keep it on," Harv said. "It'll protect you from toner."

"What's toner?" she mumbled. The words did not make it out through the mask, but Harv guessed them from the look in her eyes.

"Mites," he said, "or so they say down at the Flea Circus anyway." He picked up one of the black things taken from the mask and flicked it with a fingertip. A cineritious cloud swirled out of it, like a drop of ink in a glass of water, and hung swirling in the air, neither rising nor falling. Sparkles of light flashed in the midst of it like fairy dust. "See, there's mites around, all the time. They use the sparkles to talk to each other," Harv explained. "They're in the air, in food and water, everywhere. And there's rules that these mites are supposed to follow, and those rules are called protocols. And there's a protocol from way back that says they're supposed to be good for your lungs. They're supposed to break down into safe pieces if you breathe one inside of you." Harv paused at this point, theatrically, to summon forth one more ebon loogie, which Nell guessed must be swimming with safe mite bits. "But there are people who break those rules sometimes. Who don't follow the protocols. And I guess if there's too many mites in the air all breaking down inside your lungs, millions– well maybe those safe pieces aren't so safe if there's millions. But anyways, the guys at the Flea Circus say that sometimes the mites go to war with each other. Like maybe someone in Shanghai makes a mite that doesn't follow the protocol, and gets his matter compiler to making a whole lot of them, and sends them all across the water to New Atlantis Clave to snoop on the Vickys, or even maybe to do them harm. Then some Vicky– one of their Protocol Enforcement guys– makes a mite to go out and find that mite and kill it, and they get into a war.