But Ricky was telling me this cloud had been self-sustaining for days. That just didn't make sense. "Where is it getting power?"

He sighed. "We built the units with a small piezo wafer to generate current from photons. It's only supplementary-we added it as an afterthought-but they seem to be managing with it alone."

"So the units are solar-powered," I said.

"Right."

"Whose idea was that?"

"The Pentagon asked for it."

"And you built in capacitance?"

"Yeah. They can store charge for three hours."

"Okay, fine," I said. Now we were getting somewhere. "So they have enough power for three hours. What happens at night?"

"At night, they presumably lose power after three hours of darkness."

"And then the cloud falls apart?"

"Yes."

"And the individual units drop to the ground?"

"Presumably, yes."

"Can't you take control of them then?"

"We could," Ricky said, "if we could find them. We go out every night, looking. But we can never find them."

"You've built in markers?"

"Yes, sure. Every single unit has a fluorescing module in the shell. They show up blue-green under UV light."

"So you go out at night looking for a patch of desert that glows blue-green."

"Right. And so far, we haven't found it."

That didn't really surprise me. If the cloud collapsed tightly, it would form a clump about six inches in diameter on the desert floor. And it was a big desert out there. They could easily miss it, night after night.

But as I thought about it, there was another aspect that didn't make sense. Once the cloud fell to the ground-once the individual units lost power-then the cloud had no organization. It could be scattered by wind, like so many dust particles, never to re-form. But evidently that didn't happen. The units didn't scatter. Instead, the cloud returned day after day. Why was that?

"We think," Ricky said, "that it may hide at night."

"Hide?"

"Yeah. We think it goes to some protected area, maybe an overhang, or a hole in the ground, something like that."

I pointed to the cloud as it swirled toward us. "You think that swarm is capable of hiding?"

"I think it's capable of adapting. In fact, I know it is." He sighed. "Anyway, it's more than just one swarm, Jack."

"There's more than one?"

"There's at least three. Maybe more, by now."

I felt a momentary blankness, a kind of sleepy gray confusion that washed over me. I suddenly couldn't think, I couldn't put it together. "What are you saying?"

"I'm saying it reproduces, Jack," he said. "The fucking swarm reproduces." The camera now showed a ground-level view of the dust cloud as it swirled toward us. But as I watched, I realized it wasn't swirling like a dust devil. Instead, the particles were twisting one way, then another, in a kind of sinuous movement.

They were definitely swarming.

"Swarming" was a term for the behavior of certain social insects like ants or bees, which swarmed whenever the hive moved to a new site. A cloud of bees will fly in one direction and then another, forming a dark river in the air. The swarm might halt and cling to a tree for perhaps an hour, perhaps overnight, before continuing onward. Eventually the bees settled on a new location for their hive, and stopped swarming.

In recent years, programmers had written programs that modeled this insect behavior. Swarm-intelligence algorithms had become an important tool in computer programming. To programmers, a swarm meant a population of computer agents that acted together to solve a problem by distributed intelligence. Swarming became a popular way to organize agents to work together. There were professional organizations and conferences devoted entirely to swarm-intelligence programs. Lately it had become a kind of default solution-if you couldn't code anything more inventive, you made your agents swarm.

But as I watched, I could see this cloud was not swarming in any ordinary sense. The sinuous back-and-forth motion seemed to be only part of its movement. There was also a rhythmic expansion and contraction, a pulse, almost like breathing. And intermittently, the cloud seemed to thin out, and rise higher, then to collapse down, and become more squat. These changes occurred continuously, but in a repeating rhythm-or rather a series of superimposed rhythms. "Shit," Ricky said. "I don't see the others. And I know it's not alone." He pressed the radio again. "Vince? You see any others?"

"No, Ricky."

"Where are the others? Guys? Speak to me."

Radios crackled all over the facility. Bobby Lembeck: "Ricky, it's alone."

"It can't be alone."

Mae Chang: "Ricky, nothing else is registering out there."

"Just one swarm, Ricky." That was David Brooks.

"It can't be alone!" Ricky was gripping the radio so tightly his fingers were white. He pressed the button. "Vince? Take the PPI up to seven."

"You sure?"

"Do it."

"Well, all right, if you really think-"

"Just skip the fucking commentary, and do it!"

Ricky was talking about increasing the positive pressure inside the building to seven pounds per square inch. All clean facilities maintained a positive pressure so that outside dust particles could not enter from any leak; they would be blown outward by the escaping air. But one or two pounds was enough to maintain that. Seven pounds of positive pressure was a lot. It was unnecessary to keep out passive particles.

But of course these particles weren't passive.

Watching the cloud swirl and undulate as it came closer, I saw that parts of it occasionally caught the sunlight in a way that turned it a shimmering, iridescent silver. Then the color faded, and the swarm became black again. That had to be the piezo panels catching the sun. But it clearly demonstrated that the individual microunits were highly mobile, since the entire cloud never turned silver at the same time, but only portions, or bands. "I thought you said the Pentagon was giving up on you, because you couldn't control this swarm in wind."

"Right. We couldn't."

"But you must have had strong wind in the last few days."

"Of course. Usually comes up in late afternoon. We had ten knots yesterday."

"Why wasn't the swarm blown away?"

"Because it's figured that one out," Ricky said gloomily. "It's adapted to it."

"How?"

"Keep watching, you'll probably see it. Whenever the wind gusts, the swarm sinks, hangs near the ground. Then it rises up again once the wind dies down."

"This is emergent behavior?"

"Right. Nobody programmed it." He bit his lip. Was he lying again?

"So you're telling me it's learned ..."

"Right, right."

"How can it learn? The agents have no memory."

"Uh ... well, that's a long story," Ricky said.

"They have memory?"

"Yes, they have memory. Limited. We built it in." Ricky pressed the button on his radio. "Anybody hear anything?"

The answers came back, crackling in his handset.

"Not yet."

"Nothing."

"No sounds?"

"Not yet."

I said to Ricky, "It makes sounds?"

"We're not sure. Sometimes it seems like it. We've been trying to record it ..." He flicked keys on the workstation, quickly shifting the monitor images, making them larger, one after another. He shook his head. "I don't like this. That thing can't be alone," he said. "I want to know where the others are."

"How do you know there are others?"

"Because there always are." He chewed his lip tensely as he looked at the monitor. "I wonder what it's up to now ..."

We didn't have long to wait. In a few moments, the black swarm had come within a few yards of the building. Abruptly, it divided in two, and then divided again. Now there were three swarms, swirling side by side.

"Son of a bitch," Ricky said. "It was hiding the others inside itself." He pushed his button again. "Guys, we got all three. And they're close."

They were, in fact, too close to be seen by the ground-view camera. Ricky switched to the overhead views. I saw three black clouds, all moving laterally along the side of the building. The behavior seemed distinctly purposeful.

"What're they trying to do?" I said.

"Get inside," Ricky said.

"Why?"

"You'd have to ask them. But yesterday one of them-"

Suddenly, from a clump of cactus near the building, a cottontail rabbit sprinted away across the desert floor. Immediately, the three swarms turned and pursued it. Ricky switched the monitor view. We now watched at ground level. The three clouds converged on the terrified bunny, which was moving fast, a whitish blur on the screen. The clouds swirled after it with surprising speed. The behavior was clear: they were hunting. I felt a moment of irrational pride. PREDPREY was working perfectly! Those swarms might as well be lionesses chasing a gazelle, so purposeful was their behavior. The swarms turned sharply, then split up, cutting off the rabbit's escape to the left and right. The behavior of the three clouds clearly appeared coordinated. Now they were closing in. And suddenly one of the swarms sank down, engulfing the rabbit. The other two swarms fell on it moments later. The resulting particle cloud was so dense, it was hard to see the rabbit anymore. Apparently it had flipped onto its back, because I saw its hind legs kicking spasmodically in the air, above the cloud itself.

I said, "They're killing it ..."

"Yeah," Ricky said, nodding. "That's right."

"I thought this was a camera swarm."

"Yeah, well."

"How are they killing it?"

"We don't know, Jack. But it's fast."

I frowned. "So you've seen this before?"

Ricky hesitated, bit his lip. Didn't answer me, just stared at the screen.

I said, "Ricky, you've seen this before?"

He gave a long sigh. "Yeah. Well, the first time was yesterday. They killed a rattlesnake yesterday."

I thought, they killed a rattlesnake yesterday. I said, "Jesus, Ricky." I thought of the men in the helicopter, talking about all the dead animals. I wondered if Ricky was telling me all he knew.

"Yeah."

The rabbit no longer kicked. A single protruding foot trembled with small convulsions, and then was still. The cloud swirled low to the ground around the animal, rising and falling slightly. This continued for almost a minute.

I said, "What're they doing now?"

Ricky shook his head. "I'm not sure. But they did this before, too."

"It almost looks like they're eating it."

"I know," Ricky said.

Of course that was absurd. PREDPREY was just a biological analogy. As I watched the pulsing cloud, it occurred to me that this behavior might actually represent a program hang. I couldn't remember exactly what rules we had written for individual units after the goal was attained. Real predators, of course, would eat their prey, but there was no analogous behavior for these micro-robots. So perhaps the cloud was just swirling in confusion. If so, it should start moving again soon.

Usually, when a distributed-intelligence program stalled, it was a temporary phenomenon. Sooner or later, random environmental influences would cause enough units to act that they induced all the others to act, too. Then the program would start up again. The units would resume goal seeking.

This behavior was roughly what you saw in a lecture hall, after the lecture was over. The audience milled around for a while, stretching, talking to people close to them, or greeting friends, collecting coats and belongings. Only a few people left at once, and the main crowd ignored them. But after a certain percentage of the audience had gone, the remaining people would stop milling and begin to leave quickly. It was a kind of focus change. If I was right, then I should see something similar in the behavior of the cloud. The swirls should lose their coordinated appearance; there should be ragged wisps of particles rising into the air. Only then would the main cloud move.

I glanced at the timeclock in the corner of the monitor. "How long has it been now?"

"About two minutes."

That wasn't particularly long for a stall, I thought. At one point when we were writing PREDPREY, we used the computer to simulate coordinated agent behavior. We always restarted after a hang, but finally we decided to wait and see if the program was really permanently stalled. We found that the program might hang for as long as twelve hours before suddenly kicking off, and coming back to life again. In fact, that behavior interested the neuroscientists because-

"They're starting," Ricky said.

And they were. The swarms were beginning to rise up from the dead rabbit. I saw at once that my theory was wrong. There was no raggedness, no rising wisps. The three clouds rose up together, smoothly. The behavior seemed entirely nonrandom and controlled. The clouds swirled separately for a moment, then merged into one. Sunlight flashed on shimmering silver. The rabbit lay motionless on its side.

And then the swarm moved swiftly away, whooshing off into the desert. It shrank toward the horizon. In moments, it was gone.

Ricky was watching me. "What do you think?"

"You've got a breakaway robotic nanoswarm. That some idiot made self-powered and self-sustaining."

"You think we can get it back?"

"No," I said. "From what I've seen, there's not a chance in hell."

Ricky sighed, and shook his head.

"But you can certainly get rid of it," I said. "You can kill it."

"We can?"

"Absolutely."

"Really?" His face brightened.

"Absolutely." And I meant it. I was convinced that Ricky was overstating the problem he faced. He hadn't thought it through. He hadn't done all he could do. I was confident that I could destroy the runaway swarm quickly. I expected that I'd be done with the whole business by dawn tomorrow-at the very latest. That was how little I understood my adversary.

DAY 6

10:11 A.M.

In retrospect, I was right about one thing: it was vitally important to know how the rabbit had died. Of course I know the reason now. I also know why the rabbit was attacked. But that first day at the laboratory, I didn't have the faintest notion of what had happened. And I could never have guessed the truth.

None of us could have, at that point.

Not even Ricky.

Not even Julia.

It was ten minutes after the swarms had gone and we were all standing in the storage room. The whole group had gathered there, tense and anxious. They watched me as I clipped a radio transmitter to my belt, and pulled a headset over my head. The headset included a video camera, mounted by my left ear. It took a while to get the video transmitter working right. Ricky said, "You're really going out there?"

"I am," I said. "I want to know what happened to that rabbit." I turned to the others. "Who's coming with me?"

Nobody moved. Bobby Lembeck stared at the floor, hands in his pockets. David Brooks blinked rapidly, and looked away. Ricky was inspecting his fingernails. I caught Rosie Castro's eye. She shook her head. "No fucking way, Jack."

"Why not, Rosie?"

"You saw it yourself. They're hunting."

"Are they?"

"Sure as hell looked like it."

"Rosie," I said, "I trained you better than this. How can the swarms be hunting?"

"We all saw it." She stuck her chin out stubbornly. "All three of the swarms, hunting, coordinated."

"But how?" I said.

Now she frowned, looking confused. "What are you asking? There's no mystery. The agents can communicate. They can each generate an electrical signal."

"Right," I said. "How big a signal?"

"Well ..." She shrugged.

"How big, Rosie? It can't be much, the agent is only a hundredth of the thickness of a human hair. Can't be generating much of a signal, right?"

"True ..."

"And electromagnetic radiation decays according to the square of the radius, right?" Every school kid learned that fact in high school physics. As you moved away from the electromagnetic source, the strength faded fast-very fast.

And what that meant was the individual agents could only communicate with their immediate neighbors, with agents very close to them. Not to other swarms twenty or thirty yards away. Rosie's frown deepened. The whole group was frowning now, looking at each other uneasily.

David Brooks coughed. "Then what did we see, Jack?"

"You saw an illusion," I said firmly. "You saw three swarms acting independently, and you thought they were coordinated. But they're not. And I'm pretty certain that other things you believe about these swarms aren't true, either."

* * *

There was a lot I didn't understand about the swarms-and a lot I didn't believe. I didn't believe, for example, that the swarms were reproducing. I thought Ricky and the others must be pretty unnerved even to imagine it. After all, the fifty pounds of material they'd exhausted into the environment could easily account for the three swarms I had seen-and dozens more besides. (I was guessing that each swarm consisted of three pounds of nanoparticles. That was roughly the weight of a large bee swarm.)

As for the fact that these swarms showed purposeful behavior, that was not in the least troubling; it was the intended result of low-level programming. And I didn't believe the swarms were coordinated. It simply wasn't possible, because the fields were too weak. Nor did I believe the swarms had the adaptive powers that Ricky attributed to them. I'd seen too many demos of robots carrying out some task-like cooperating to push a box around the room-which was interpreted by observers as intelligent behavior, when in fact the robots were stupid, minimally programmed, and cooperating by accident. A lot of behavior looked smarter than it was. (As Charley Davenport used to say, "Ricky should thank God for that.") And finally, I didn't really believe that the swarms were dangerous. I didn't think that a three-pound cloud of nanoparticles could represent much of a threat to anything, not even a rabbit. I wasn't at all sure it had been killed. I seemed to recall that rabbits were nervous creatures, prone to die of fright. Or the pursuing particles might have swarmed in through the nose and mouth, blocking the respiratory passages and choking the animal to death. If so, the death was accidental, not purposeful. Accidental death made more sense to me. In short, I thought that Ricky and the others had consistently misinterpreted what they saw. They'd spooked themselves.

On the other hand, I had to admit that several unanswered questions nagged at me. The first, and most obvious, was why the swarm had escaped their control. The original camera swarm was designed to be controlled by an RF transmitter beaming toward it. Now apparently the swarm ignored transmitted radio commands, and I didn't understand why. I suspected an error in manufacturing. The particles had probably been made incorrectly. Second was the question of the swarm's longevity. The individual particles were extremely small, subject to damage from cosmic rays, photochemical decay, dehydration of their protein chains, and other environmental factors. In the harsh desert, all the swarms should have shriveled up and died of "old age" many days ago. But they hadn't. Why not? Third, there was the problem of the swarm's apparent goal. According to Ricky, the swarms kept coming back to the main building. Ricky believed they were trying to get inside. But that didn't seem to be a reasonable agent goal, and I wanted to look at the program code to see what was causing it. Frankly, I suspected a bug in the code. And finally, I wanted to know why they had pursued the rabbit. Because PREDPREY didn't program units to become literal predators. It merely used a predator model to keep the agents focused and goal-oriented. Somehow, that had changed, and the swarms now appeared to be actually hunting.

That, too, was probably a bug in the code.

To my mind, all these uncertainties came down to a single, central question-how had the rabbit died? I didn't think it had been killed. I suspected the rabbit's death was accidental, not purposeful.

But we needed to find out.

I adjusted my portable radio headset, with the sunglasses and the video camera mounted by the left eye. I picked up the plastic bag for the rabbit's body and turned to the others. "Anybody coming with me?"

There was an uncomfortable silence.

Ricky said, "What's the bag for?"

"To bring the rabbit back in."

"No fucking way," Ricky said. "You want to go out there, that's your business. But you're not bringing that rabbit back here."

"You've got to be kidding," I said.

"I'm not. We run a level-six clean environment here, Jack. That rabbit's filthy. Can't come in."

"All right, then, we can store it in Mae's lab and-"

"No way, Jack. Sorry. It's not coming through the first airlock." I looked at the others. They were all nodding their heads in agreement.

"All right, then. I'll examine it out there."

"You're really going to go out?"

"Why not?" I looked at them, one after another. "I have to tell you guys, I think you've all got your knickers in a twist. The cloud's not dangerous. And yes, I'm going out." I turned to Mae. "Do you have a dissection kit of some kind that-"

"I'll come with you," she said quietly.

"Okay. Thanks." I was surprised that Mae was the first to come around to my way of seeing things. But as a field biologist, she was probably better than the others at assessing real-world risk. In any case, her decision seemed to break some tension in the room; the others visibly relaxed. Mae went off to get the dissecting tools and some lab equipment. That was when the phone rang. Vince answered it, and turned to me. "You know somebody named Dr. Ellen Forman?"

"Yes." It was my sister.

"She's on the line." Vince handed me the phone, and stepped back. I felt suddenly nervous. I glanced at my watch. It was eleven o'clock in the morning, time for Amanda's morning nap. She should be asleep in her crib by now. Then I remembered I had promised my sister I would call her at eleven to check in, to see how things were going. I said, "Hello? Ellen? Is everything all right?"

"Sure. Fine." A long, long sigh. "It's fine. I don't know how you do it, is all."

"Tired?"

"About as tired as I've ever felt."

"Kids get off to school okay?"

Another sigh. "Yes. In the car, Eric hit Nicole on the back, and she punched him on the ear."

"You've got to interrupt them if they start that, Ellen."

"So I'm learning," she said wearily.

"And the baby? How's her rash?"

"Better. I'm using the ointment."

"Her movements okay?"

"Sure. She's well coordinated for her age. Is there a problem I should know about?"

"No, no," I said. I turned away from the group, lowered my voice. "I meant, is she pooping okay?"

Behind me, I heard Charley Davenport snicker.

"Copiously," Ellen said. "She's sleeping now. I took her to the park for a while. She was ready to go down. Everything's okay at the house. Except the pilot for the water heater went out, but the guy's coming to fix it."

"Good, good ... Listen, Ellen, I'm in the middle of something here-"

"Jack? Julia called from the hospital a few minutes ago. She was looking for you."

"Uh-huh ..."

"When I said you'd gone to Nevada, she got pretty upset."

"Is that right?"

"She said you didn't understand. And you were going to make it worse. Something like that. I think you better call her. She sounded agitated."

"Okay. I'll call."

"How are things going out there? You be back tonight?"

"Not tonight," I said. "Sometime tomorrow morning. Ellen, I have to go now-"

"Call the kids at dinnertime, if you can. They'd like to hear from you. Auntie Ellen is fine, but she's not Dad. You know what I mean."

"Okay. You'll eat at six?"

"About."

I told her I'd try to call, and I hung up.

* * *

Mae and I were standing by the double glass walls of the outer airlock, just inside the building entrance. Beyond the glass, I could see the solid-steel fire door that led outside. Ricky was standing beside us, gloomy and nervous, watching as we made our final preparations. "You sure this is necessary? To go outside?"

"It's essential."

"Why don't you and Mae wait until nightfall, and go out then?"

"Because the rabbit won't be there," I said. "By nightfall, coyotes or hawks will have come and taken the carcass away."

"I don't know about that," Ricky said. "We haven't seen any coyotes around here for a while."

"Oh hell," I said impatiently, turning on my radio headset. "In the time we've spent arguing about this, we could have been out and back already. See you, Ricky." I went through the glass door, and stood in the airlock. The door hissed shut behind me. The air handlers whooshed briefly in the now-familiar pattern, and then the far glass slid open. I walked toward the steel fire door. Looking back, I saw Mae stepping into the airlock. I opened the fire door a crack. Harsh, glaring sunlight laid a burning strip on the floor. I felt hot air on my face. Over the intercom, Ricky said, "Good luck, guys." I took a breath, pushed the door wider, and stepped out into the desert. The wind had dropped, and the midmorning heat was stifling. Somewhere a bird chittered; otherwise it was silent. Standing by the door, I squinted in the glare of the sunlight. A shiver ran down my back. I took another deep breath.

I was certain that the swarms were not dangerous. But now that I was outside, my theoretical inferences seemed to lose force. I must have caught Ricky's tension, because I was feeling distinctly uneasy. Now that I was outside, the rabbit carcass looked much farther away than I had imagined. It was perhaps fifty yards from the door, half the length of a football field. The surrounding desert seemed barren and exposed. I scanned the shimmering horizon, looking for black shapes. I saw none.