"I'm taking it easy," I said. "Which phone? In the main unit?"

"Jack." She put her hands on my shoulders, stopped me. "It's after ten o'clock. Forget it."

"Forget it? He could have gotten us killed."

"And right now we have work to do."

I looked at her calm face, her steady expression. I thought of the swift way she had eviscerated the rabbit.

"You're right," I said.

"Good," she said, turning away. "Now I think as soon as we get some backpacks, we'll be ready to go."

There was a reason, I thought, why Mae never lost an argument. I went to the storage cupboard and got out three packs. I threw one to Bobby.

"Let's hit the road," I said.

It was a clear night, filled with stars. We walked in darkness toward the storage shed, a dark outline against the dark sky. I pushed the dirt bike along. None of us talked for a while. Finally, Bobby said, "We're going to need lights."

"We're going to need a lot of things," Mae said. "I made a list." We came to the storage shed, and pushed open the door. I saw Bobby hang back in the darkness. I went in, and fumbled for the lights. I flicked them on. The interior of the storage shed appeared just as we had left it. Mae unzipped her backpack and began walking down the row of shelves. "We need portable lights ... ignition fuses ... flares ... oxygen ..."

Bobby said, "Oxygen? Really?"

"If this site is underground, yes, we may ... and we need thermite."

I said, "Rosie had it. Maybe she set it down when she ... I'll look." I went into the next room. The box of thermite tubes lay overturned on the floor, the tubes nearby. Rosie must have dropped it when she ran. I wondered if she had had any in her hand. I looked over at her body by the door.

Rosie's body was gone.

"Jesus."

Bobby came running in. "What is it? What's wrong?"

I pointed to the door. "Rosie's gone."

"What do you mean, gone?"

I looked at him. "Gone, Bobby. The body was here before and now it's gone."

"How can that be? An animal?"

"I don't know." I went over and crouched down at the spot where her body had been. When I had last seen her, five or six hours ago, her body had been covered with a milky secretion. Some of that secretion covered the floor, too. It looked exactly like thick, dried milk. Up where her head had been, the secretion was smooth and undisturbed. But closer to the door, it appeared to have been scraped. There were streaks in the coating. "It looks like she was dragged out," Bobby said.

"Yes."

I peered closely at the secretion, looking for footprints. A coyote alone couldn't have dragged her; a pack of animals would be needed to pull her out the door. They would surely leave marks. I saw none.

I got up and walked to the door. Bobby stood beside me, looking out into the darkness.

"You see anything?" he said.

"No."

I returned to Mae. She had found everything. She had coiled magnesium fuse. She had flare guns. She had portable halogen flashlights. She had head-mounted lamps with big elastic bands. She had small binoculars and night-vision goggles. She had a field radio. And she had oxygen bottles and clear-plastic gas masks. I was uneasy when I recognized that these were the same plastic masks I had seen on the men in the SSVT van back in California last night, except they weren't silvered.

And then I thought, Was it only last night? It was. Hardly twenty-four hours had passed.

It felt to me like a month.

Mae was dividing everything into the three backpacks. Watching her, I realized that she was the only one of us with actual field experience. In comparison, we were all stay-at-homes, theoreticians. I was surprised how dependent on her I felt tonight. Bobby hefted the nearest pack and grunted. "You really think we need all this stuff, Mae?"

"It's not like you have to carry it; we're driving. And yes, better safe than sorry."

"Okay, fine, but I mean-a field radio?"

"You never know."

"Who you gonna call?"

"The thing is, Bobby," she said, "if it turns out you need any of this stuff, you really need it."

"Yeah, but it's-"

Mae picked up the second backpack, and slung it over her shoulder. She handled the weight easily. She looked at Bobby. "You were saying?"

"Never mind."

I picked up the third backpack. It wasn't bad. Bobby was complaining because he was scared. It was true that the oxygen bottle was a little larger and heavier than I would have liked, and it fitted awkwardly into the backpack. But Mae insisted we have extra oxygen. Bobby said nervously, "Extra oxygen? How big do you guys think this hiding place is?"

"I have no idea," Mae said. "But the most recent swarms are much larger." She went to the sink, and picked up the radiation counter. But when she unplugged it from the wall, she saw the battery was dead. We had to hunt for a new battery, unscrew the case, replace the battery. I was worried the replacement would be dead, too. If it was, we were finished.

Mae said, "We better be careful with the night-vision goggles, too. I don't know how good any of the batteries are for the stuff we have."

But the counter clicked loudly. The battery indicator glowed. "Full power," she said. "It'll last four hours."

"Let's get started," I said.

It was 10:43 P.M.

The radiation counter went crazy when we came to the Toyota, clicking so rapidly the sound was continuous. Holding the wand in front of her, Mae left the car, walked into the desert. She turned west and the clicks diminished. She went east and they picked up again. But as she continued east, the clicks slowed. She turned north, and they increased. "North," she said.

I got on the bike, gunned the engine.

Bobby rumbled out of the shed on the All-Terrain Vehicle, with its fat rear tires and bicycle handlebars. The ATV looked ungainly but I knew it was probably better suited to night travel in the desert.

Mae got on the back of my bike, leaned over to hold the wand near the ground, and said, "Okay. Let's go."

We started off into the desert, under a cloudless night sky.

The headlight on the bike bounced up and down, jerking the shadows on the terrain ahead, making it difficult to see what was coming. The desert that had looked so flat and featureless in daylight was now revealed to have sandy dips, rock-filled beds, and deep arroyos that came up without warning. It took all my attention to keep the bike upright-particularly since Mae was continuously calling to me, "Go left ... now right ... now right ... okay, too much, left ..." Sometimes we had to make a full circle until she could be certain of the right path. If anybody followed our track in daylight, they'd think the driver must be drunk, it twisted and turned so much. The bike jumped and swerved on rough ground. We were now several miles from the lab, and I was starting to worry. I could hear the counter clicks, and they were becoming less frequent. It was getting hard to distinguish the swarm trail from the background radiation. I didn't understand why that should happen but there was no question it was. If we didn't locate the swarm hiding place soon, we'd lose the trail entirely. Mae was worried, too. She kept bending over closer and closer to the ground, with one hand on the wand and one hand around my waist. And I had to go slower, because the trail was becoming so faint. We lost the trail, found it, went off it again. Under the black canopy of stars, we backtracked, turned in circles. I caught myself holding my breath. And at last I was going around and around in the same spot, trying not to feel desperate. I made the circle three times, then four, but to no avail: the counter in Mae's hand just clicked randomly. And suddenly it was clear to us that the trail was truly lost. We were out here in the middle of nowhere, driving in circles.

We had lost the trail.

Exhaustion hit me suddenly, and hard. I had been running on adrenaline all day and now that I was finally defeated a deep weariness came over my body. My eyes drooped. I felt as if I could go to sleep standing on the bike.

Behind me, Mae sat up and said, "Don't worry, okay?"

"What do you mean?" I said wearily. "My plan has totally failed, Mae."

"Maybe not yet," she said.

Bobby pulled up close to us. "You guys look behind you?" he said.

"Why?"

"Look back," he said. "Look how far we've come."

I turned and looked over my shoulder. To the south, I saw the bright lights of the fabrication building, surprisingly close. We couldn't be more than a mile or two away. We must have traveled in a big semicircle, eventually turning back toward our starting point. "That's weird."

Mae had got off the bike, and stepped in front of the headlamp. She was looking at the LCD readout on the counter. She said, "Hmmm."

Bobby said hopefully, "So, what do you say, Mae? Time to go back?"

"No," Mae said. "It's not time to go back. Take a look at this." Bobby leaned over, and we both looked at the LCD readout. It showed a graph of radiation intensity, stepping progressively downward, and finally dropping quickly. Bobby frowned. "And this is?"

"Time course of tonight's readings," she said. "The machine's showing us that ever since we started, the intensity of the radiation has declined arithmetically-it's a straight-line decrease, a staircase, see there? And it's stayed arithmetic until the last minute or so, when the decrease suddenly became exponential. It just fell to zero."

"So?" Bobby looked puzzled. "That means what? I don't get it."

"I do." She turned to me, climbed back on the bike. "I think I know what happened. Go forward-slowly."

I let out the clutch, and rumbled forward. My bouncing headlight showed a slight rise in the desert, scrubby cactus ahead ...

"No. Slower, Jack."

I slowed. Now we were practically going at a walk. I yawned. There was no point in questioning her; she was intense, focused. I was just tired and defeated. We continued up the desert rise until it flattened, and then the bike began to tilt downward-

"Stop."

I stopped.

Directly ahead, the desert floor abruptly ended. I saw blackness beyond.

"Is that a cliff?"

"No. Just a high ridge."

I edged the bike forward. The land definitely fell away. Soon we were at the edge and I could get my bearings. We were at the crest of a ridge fifteen feet high, which formed one side of a very wide streambed. Directly beneath me I saw smooth river rocks, with occasional boulders and clumps of scraggly brush that stretched about fifty yards away, to the far side of the riverbed. Beyond the distant bank, the desert was flat again. "I understand now," I said. "The swarm jumped."

"Yes," she said, "it became airborne. And we lost the trail."

"But then it must have landed somewhere down there," Bobby said, pointing to the streambed.

"Maybe," I said. "And maybe not."

I was thinking it would take us many minutes to find a safe route down. Then we would spend a long time searching among the bushes and rocks of the streambed, before picking up the trail again. It might take hours. We might not find it at all. From our position up here on top of the ridge, we saw the daunting expanse of desert stretching out before us. I said, "The swarm could have touched down in the streambed. Or it could have come down just beyond the bed. Or it could have gone quarter mile beyond." Mae was not discouraged. "Bobby, you stay here," she said. "You'll mark the position where it jumped. Jack and I will find a path down, go out into that plain, and run in a straight line east-west until we pick up the trail again. Sooner or later, we'll find it."

"Okay," Bobby said. "Got you."

"Okay," I said. We might as well do it. We had nothing to lose. But I had very little confidence we would succeed.

Bobby leaned forward over his ATV. "What's that?"

"What?"

"An animal. I saw glowing eyes."

"Where?"

"In that brush over there." He pointed to the center of the streambed. I frowned. We both had our headlights trained down the ridge. We were lighting a fairly large arc of desert. I didn't see any animals.

"There!" Mae said.

"I don't see anything."

She pointed. "It just went behind that juniper bush. See the bush that looks like a pyramid? That has the dead branches on one side?"

"I see it," I said. "But ..." I didn't see an animal.

"It's moving left to right. Wait a minute and it'll come out again." We waited, and then I saw a pair of bright green, glowing spots. Close to the ground, moving right. I saw a flash of pale white. And almost immediately I knew that something was wrong. So did Bobby. He twisted his handlebars, moving his headlamp to point directly to the spot. He reached for binoculars.

"That's not an animal ..." he said.

Moving among the low bushes, we saw more white-flesh white. But we saw only glimpses. And then I saw a flat white surface that I realized with a shock was a human hand, dragging along the ground. A hand with outstretched fingers.

"Jesus," Bobby said, staring through the binoculars.

"What? What is it?"

"It's a body being dragged," he said. And then, in a funny voice, he said, "It's Rosie."

DAY 6

10:58 P.M.

Gunning the bike, I took off with Mae, running along the edge of the ridge until it sloped down toward the streambed floor. Bobby stayed where he was, watching Rosie's body. In a few minutes I had crossed the streambed to the other bank, and was moving back toward his light on the hill.

Mae said, "Let's slow down, Jack."

So I slowed down, leaning forward over my handlebars, trying to see the ground far ahead. Suddenly the radiation counter began to chatter again.

"Good sign," I said.

We moved ahead. Now we were directly across from Bobby on the ridge above. His headlamp cast a faint light on the ground all around us, sort of like moonlight. I waved for him to come down. He turned his vehicle and headed west. Without his light, the ground was suddenly darker, more mysterious.

And then we saw Rosie Castro.

Rosie lay on her back, her head tilted so she appeared to be looking backward, directly at me, her eyes wide, her arm outstretched toward me, her pale hand open. There was an expression of pleading-or terror-on her face. Rigor mortis had set in, and her body jerked stiffly as it moved over low shrubs and desert cactus.

She was being dragged away-but no animal was dragging her.

"I think you should turn your light off," Mae said.

"But I don't see what's doing it ... there's like a shadow underneath her ..."

"That's not a shadow," Mae said. "It's them."

"They're dragging her?"

She nodded. "Turn your light off."

I flicked off the headlamp. We stood in darkness. I said, "I thought swarms couldn't maintain power more than three hours."

"That's what Ricky said."

"He's lying again?"

"Or they've overcome that limitation in the wild."

The implications were unsettling. If the swarms could now sustain power through the night, then they might be active when we reached their hiding place. I was counting on finding them collapsed, the particles spread on the ground. I intended to kill them in their sleep, so to speak. Now it seemed they weren't sleeping.

We stood there in the cool dark air, thinking things over. Finally Mae said, "Aren't these swarms modeled on insect behavior?"

"Not really," I said. "The programming model was predator-prey. But because the swarm is a population of interacting particles, to some degree it will behave like any population of interacting particles, such as insects. Why?"

"Insects can execute plans that take longer than the lifespan of a single generation. They can build nests that require many generations. Isn't that true?"

"I think so ..."

"So maybe one swarm carried the body for a while, and then another took over. Maybe there have been three or four swarms so far. That way none of them has to go three hours at night." I didn't like the implications of that idea any better. "That would mean the swarms are working together," I said. "It would mean they're coordinated."

"They clearly are, by now."

"Except that's not possible," I said to her. "Because they don't have the signaling capability."

"It wasn't possible a few generations ago," Mae said. "Now it is. Remember the V formation that came toward you? They were coordinated."

That was true. I just hadn't realized it at the time. Standing there in the desert night, I wondered what else I hadn't realized. I squinted into the darkness, trying to see ahead. "Where are they taking her?" I said.

Mae unzipped my backpack, and pulled out a set of night goggles. "Try these." I was about to help her get hers, but she'd deftly taken her pack off, opened it, and pulled out her own goggles. Her movements were quick, sure.

I slipped on the headset, adjusted the strap, and flipped the lenses down over my eyes. These were the new Gen 4 goggles that showed images in muted color. Almost immediately, I saw Rosie in the desert. Her body was disappearing behind the scrub as she moved farther and farther away.

"Okay, so where are they taking her?" I said again. Even as I spoke, I raised the goggles higher, and at once I saw where they were taking her.

From a distance it looked like a natural formation-a mound of dark earth about fifteen feet wide and six feet high. Erosion had carved deep, vertical clefts so that the mound looked a little like a huge gear turned on edge. It would be easy to overlook this formation as natural. But it wasn't natural. And erosion hadn't produced its sculpted look. On the contrary, I was seeing an artificial construction, similar to the nests made by African termites and other social insects.

Wearing the second pair of goggles, Mae looked for a while in silence, then said, "Are you going to tell me that is the product of self-organized behavior? That the behavior to make it just emerged all by itself?"

"Actually, yes," I said. "That's exactly what happened."

"Hard to believe."

"I know."

Mae was a good biologist, but she was a primate biologist. She was accustomed to studying small populations of highly intelligent animals that had dominance hierarchies and group leaders. She understood complex behavior to be the result of complex intelligence. And she had trouble grasping the sheer power of self-organized behavior within a very large population of dumb animals.

In any case, this was a deep human prejudice. Human beings expected to find a central command in any organization. States had governments. Corporations had CEOs. Schools had principals. Armies had generals. Human beings tended to believe that without central command, chaos would overwhelm the organization and nothing significant could be accomplished. From this standpoint, it was difficult to believe that extremely stupid creatures with brains smaller than pinheads were capable of construction projects more complicated than any human project. But in fact, they were.

African termites were a classic example. These insects made earthen castlelike mounds a hundred feet in diameter and thrusting spires twenty feet into the air. To appreciate their accomplishment, you had to imagine that if termites were the size of people, these mounds would be skyscrapers one mile high and five miles in diameter. And like a skyscraper, the termite mound had an intricate internal architecture to provide fresh air, remove excess CO2and heat, and so on. Inside the structure were gardens to grow food, residences for royalty, and living space for as many as two million termites. No two mounds were exactly the same; each was individually constructed to suit the requirements and advantages of a particular site. All this was accomplished with no architect, no foreman, no central authority. Nor was a blueprint for construction encoded in the termite genes. Instead these huge creations were the result of relatively simple rules that the individual termites followed in relation to one another. (Rules like, "If you smell that another termite has been here, put a dirt pellet on this spot.") Yet the outcome was arguably more complex than any human creation. Now we were seeing a new construction made by a new creature, and it was again difficult to conceive how it might have been made. How could a swarm make a mound, anyway? But I was beginning to realize that out here in the desert, asking how something happened was a fool's errand. The swarms were changing fast, almost minute to minute. The natural human impulse to figure it out was a waste of time. By the time you figured it out, things would have changed.

Bobby rumbled up in his ATV, and cut his light. We all stood there under the stars. Bobby said, "What do we do now?"

"Follow Rosie," I said.

"Looks like Rosie is going into that mound," he said. "You mean we follow her there?"

"Yes," I said.

At Mae's suggestion, we walked the rest of the way. Lugging our backpacks, it took us the better part of ten minutes to reach the vicinity of the mound. We paused about fifty feet away. There was a nauseating smell in the air, a putrid odor of rotting and decay. It was so strong it made my stomach turn. Then too, a faint green glow seemed to be emanating from inside the mound.

Bobby whispered, "You really want to go in there?"

"Not yet," Mae whispered. She pointed off to one side. Rosie's body was moving up the slope of the mound. As she came to the rim, her rigid legs pointed into the air for a moment. Then her body toppled over, and she fell into the interior. But she stopped before she disappeared entirely; for several seconds, her head remained above the rim, her arm outstretched, as if she were reaching for air. Then, slowly, she slid the rest of the way down, and vanished. Bobby shivered.

Mae whispered, "Okay. Let's go."

She started forward in her usual noiseless way. Following her, I tried to be as quiet as I could. Bobby crunched and crackled his way along the ground. Mae paused, and gave him a hard look.

Bobby held up his hands as if to say, what can I do?

She whispered, "Watch where you put your feet."

He whispered, "I am."

"You're not."

"It's dark, I can't see."

"You can if you try."

I couldn't recall ever seeing Mae show irritation before, but we were all under pressure now. And the stench was terrible. Mae turned and once again moved forward silently. Bobby followed, making just as much noise as before. We had only gone a few steps before Mae turned, held up her hand, and signaled for him to stay where he was. He shook his head, no. He clearly didn't want to be left alone.

She gripped his shoulder, pointed firmly to the ground, and whispered, "You stay here."

"No ..."

She whispered, "You'll get us all killed."

He whispered, "I promise."

She shook her head, pointed to the ground. Sit.

Finally, Bobby sat down.

Mae looked at me. I nodded. We set out again. By now we were twenty feet from the mound itself. The smell was almost overpowering. My stomach churned; I was afraid I might be sick. And this close, we began to hear the deep thrumming sound. More than anything it was that sound that made me want to run away. But Mae kept going.

We crouched down as we climbed the mound, and then lay flat along the rim. I could see Mae's face in the green glow coming from inside. For some reason the stench didn't bother me anymore. Probably because I was too frightened.

Mae reached into the side pouch of her pack, and withdrew a small thumb-sized camera on a thin telescoping stick. She brought out a tiny LCD screen and set it on the ground between us. Then she slid the stick over the rim.

On the screen, we saw a green interior of smooth undulating walls. Nothing seemed to be moving. She turned the camera this way and that. All we saw were green walls. There was no sign of Rosie.

Mae looked at me, pointed to her eyes. Want to take a look now?

I nodded.

We inched forward slowly, until we could look over the rim.

It wasn't what I expected at all.

The mound simply narrowed an existing opening that was huge-twenty feet wide or more, revealing a rock slide that sloped downward from the rim and ended at a gaping hole in the rock to our right. The green light was coming from somewhere inside this gaping hole. What I was seeing was the entrance to a very large cave. From our position on the rim, we couldn't see into the cave itself, but the thrumming sound suggested activity within. Mae opened the telescoping stick to its full length, and gently lowered the camera into the hole. Soon we could see farther into the cave. It was undoubtedly natural, and large: perhaps eight feet high, ten feet wide. The rock walls were pale white, and appeared to be covered with the milky substance we'd seen on Rosie.

And Rosie's body was only a short distance inside. We could see her hand sticking out around a bend in the rock wall. But we could see nothing beyond the bend. Mae signaled me: want to go down?

I nodded slowly. I didn't like how this felt, I didn't like that I had no idea what was beyond the bend. But we really had no choice.

She pointed back toward Bobby. Get him?

I shook my head, no. He wouldn't help us here.

She nodded, and started very slowly to slide out of her backpack, making no sound at all, when she suddenly froze. Literally froze: she didn't move a muscle. I looked at the screen. And I froze, too.

A figure had walked from behind the bend, and now stood alertly at the entrance of the cave, looking around.

It was Ricky.

He was behaving as if he had heard a sound, or had been alerted for some other reason. The video camera still dangled down the rim of the mound. It was pretty small; I didn't know if he would see it.

I watched the screen tensely.

The camera didn't have good resolution and the screen was the size of my palm, but it was still clear that the figure was Ricky. I didn't understand what he was doing here-or even how he had gotten here. Then another man came around the bend.

He was also Ricky.

I glanced at Mae, but she remained utterly still, a statue. Only her eyes moved. I squinted at the screen. Within the limits of video resolution, the two figures appeared to be identical in every respect. Same clothes, same movements, same gestures and shrugs. I couldn't see the faces well, but I had the impression they were more detailed than before. They didn't seem to notice the camera.

They looked up at the sky, and then at the rock slide for a while, and then they turned their backs on us, and returned to the interior of the cave.

Still Mae did not move. She had been motionless for almost a minute already and in that time she hadn't even blinked. Now the men were gone, and-

Another figure came around the corner. It was David Brooks. He moved awkwardly, stiffly at first, but he quickly became more fluid. I had the feeling I was watching a puppeteer perfect his moves, animating the figure in a more lifelike way. Then David became Ricky. And then David again. And the David figure turned and went away.

Still Mae waited. She waited fully two more minutes, and then finally withdrew the camera. She jerked her thumb, indicating we should go back. Together, we crept away from the rim, back down the mound, and moved away silently into the desert night. We gathered a hundred yards to the west, near our vehicles. Mae was rummaging in her backpack; she pulled out a clipboard with a felt marker. She flicked on her penlight and began to draw.

"This is what you're up against," she said. "The cave has an opening like this, which you saw. Past the bend, there's a big hole in the floor, and the cave spirals downward for maybe a hundred yards. That brings you into one large chamber that is maybe a hundred feet high, and a couple of hundred feet wide. Single big room, that's all. There are no passages leading off, at least none that I saw."

"That you saw?"

"I've been in there," she said, nodding.

"When?"

"A couple of weeks ago. Back when we first started looking for the swarm's hiding place. I found that cave and went in there in day-light. I didn't find any indication of a swarm then." She explained that the cave was filled with bats, the whole ceiling covered with them, packed together in a pink squirming mass, all the way out to the entrance. "Ugh," Bobby said. "I hate bats."

"I didn't see any bats there tonight."

"You think they've been driven away?"

"Eaten, probably."

"Jesus, guys," Bobby said, shaking his head. "I'm just a programmer. I don't think I can do this. I don't think I can go in there."

Mae ignored him. She said to me, "If we go in," she said, "we'll have to set off thermite, and keep doing it all the way down to the chamber. I'm not sure we have enough thermite to do that."

"Maybe not," I said. I had a different concern. "We're wasting our time unless we destroy all the swarms, and all the assemblers that are making them. Right?" They both nodded.

"I'm not sure that'll be possible," I said. "I thought the swarms would be powered down at night. I thought we could destroy them on the ground. But they're not powered down-at least not all of them. And if just one of them gets past us, if it escapes from the cave ..." I shrugged. "Then this has all been a waste of time."

"Right," Bobby said, nodding. "That's right. It'd be a waste of time."

Mae said, "We need some way to trap them in the cave."

"There isn't any way," Bobby said. "I mean, they can just fly out, whenever they want." Mae said, "There might be a way." She started rummaging in her backpack again, looking for something. "Meanwhile, the three of us better spread out."

"Why?" Bobby said, alarmed.