"All true," Ricky said. "Except we can't. And we've tried everything we can think of." He was focused on the screen, watching intently. "That cloud is independent of us. Period."

"And so you brought me out here…"

"To help us get the fucking thing back," Ricky said.

It was, I thought, a problem no one had ever imagined before. In all the years that I had been programming agents, the focus had been on getting them to interact in a way that produced useful results. It never occurred to us that there might be a larger control issue, or a question of independence. Because it simply couldn't happen. Individual agents were too small to be self-powered; they had to get their energy from some external source, such as a supplied electrical or microwave field. All you had to do was turn off the field, and the agents died. The swarm was no more difficult to control than a household appliance, like a kitchen blender. Flip the power off and it went dead.

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.