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.