"No," he said, conscious that he could almost see the shape of something; something that was close to the truth if not the truth itself. "There"s another possibility, one I hadn"t thought of until now."

Galiana squinted at him, that rare frown appearing. "Which is?"

"That someone else removed his helmet for him."

They went down into the bowels of the base. In the dead s.p.a.ce of the equipment bays Galiana became ill at ease. She was not used to being out of communicational range of her colleagues. Normally systems buried in the environment picked up neural signals from individuals, amplifying and rebroadcasting them to other people, but there were no such systems here. Clavain could hear Galiana"s thoughts, but they came in weakly, like a voice from the sea almost drowned by the roar of surf. "This had better be worth it," Galiana said.

"I want to show you the airlock," Clavain answered. "I"m sure Setterholm must have left here with his helmet properly attached."

"You still think he was murdered?"

"I think it"s a remote possibility that we should be very careful not to discount."

"But why would anyone kill a man whose only interest was a lot of harmless ice worms?"

"That"s been bothering me as well."

"And?"

"I think I have an answer. Half of one, anyway What if his interest in the worms brought him into conflict with the others? I"m thinking about the reactor."

Galiana nodded. "They"d have needed to harvest ice for it."

"Which Setterholm might have seen as interfering with the worms" ecology. Maybe he made a nuisance of himself and someone decided to get rid of him."

"That would be a pretty extreme way of dealing with him."

"I know," Clavain said, stepping through a connecting door into the transport bay. "I said I had half an answer, not all of one."

As soon as he was through he knew something was amiss. The bay was not as it had been before, when he had come down here scouting for clues. He dropped his train of thought immediately, focusing only on the now.

The room was much, much colder than it should have been. And lighter. There was an oblong of chill blue daylight spilling across the floor, from the huge open door of one of the vehicle exit ramps. Clavain looked at it in mute disbelief, wanting it to be a temporary glitch in his vision. But Galiana was with him, and she had seen it too.

"Someone"s left the base," she said.

Clavain looked out across the ice. He could see the wake which the vehicle had left in the snow, arcing out toward the horizon. For a long moment they stood at the top of the ramp, frozen into inaction.

Clavain"s mind screamed with the implications. He had never really liked the idea of Iverson taking Felka away with him elsewhere in the base, but he had never considered the possibility that he might take her into one of the blind zones. From here, Iverson must have known enough little tricks to open a surface door, start a rover and leave, without any of the Conjoiners realizing.

"Nevil, listen to me," Galiana said. "He doesn"t necessarily mean her any harm. He might just want to show her something."

He turned to her. "There isn"t time to arrange a shuttle. That trick you did a few days ago-talking to the door? Do you think you can manage it again?"

"I don"t need to. The door"s already open."

Clavain nodded at one of the other rovers, hulking behind them. "It"s not the door I"m thinking about."

Galiana was disappointed; it took her three minutes to convince the machine to start, rather than the few dozen seconds she said it should have taken. She was, she told Clavain, in serious danger of getting rusty at this sort of thing. Clavain just thanked the G.o.ds that there had been no mechanical sabotage to the rover; no amount of neural intervention could have fixed that.

"That"s another thing that makes it look like this is just an innocent trip outside," Galiana said. "If he"d really wanted to abduct her, it wouldn"t have taken much additional effort to stop us following him. If he"d closed the door, as well, we might not even have noticed he was gone."

"Haven"t you ever heard of reverse psychology?" Clavain said.

"I still can"t see Iverson as a murderer, Nevil." She checked his expression, her own face calm despite her driving the machine. Her hands were folded in her lap. She was less isolated now, having used the rover"s comm systems to establish a link back to the other Conjoiners. "Setterholm, maybe. The obsessive loner and all that. Just a shame he"s the dead one."

"Yes," Clavain said, uneasily.

The rover itself ran on six wheels; a squat, pressurized hull perched low between absurd-looking balloon tires. Galiana gunned them hard down the ramp and across the ice, trusting the machine to glideharmlessly over the smaller creva.s.ses. It seemed reckless, but if they followed the trail that Iverson had left, they were almost guaranteed not to hit any fatal obstacles.

"Did you get anywhere with the source of the sickness?" Clavain asked.

"No breakthroughs yet..."

"Then here"s a suggestion. Can you read my visual memory accurately?" Clavain did not need an answer. "While you were finding Iverson"s body, I was looking over the lab samples. There were a lot of terrestrial organisms there. Could one of those have been responsible?"

"You"d better replay the memory."

Clavain did; picturing himself looking over the rows of culture dishes, test-tubes and gel-slides; concentrating especially on those that had come from Earth rather than the locally obtained samples. In his mind"s eye the sample names refused to snap into clarity, but the machines which Galiana had seeded through his mind would already be locating the eidetically stored short-term memories and retrieving them with a clarity beyond the capabilities of Clavain"s own brain.

"Now see if there"s anything there which might do the job."

"A terrestrial organism?" Galiana sounded surprised. "Well, there might be something there, but I can"t see how it could have spread beyond the laboratory unless someone wanted it to."

"I think that"s exactly what happened."

"Sabotage?"

"Yes."

"Well, we"ll know sooner or later. I"ve pa.s.sed the information to the others. They"ll get back to me if they find a candidate. But I still don"t see why anyone would sabotage the entire base, even if it was possible. Overthrowing the von Neumann machines is one thing ... ma.s.s suicide is another."

"I don"t think it was ma.s.s suicide. Ma.s.s murder, maybe."

"And Iverson"s your main suspect?"

"He survived, didn"t he? And Setterholm scrawled a message in the ice just before he died. It must have been a warning about him." But even as he spoke, he knew there was a second possibility; one that he could not quite focus on.

Galiana swerved the rover to avoid a particularly deep and yawning chasm, shaded with vivid veins of turquoise blue.

"There"s a small matter of missing motive."

Clavain looked ahead, wondering if the thing he saw glinting in the distance was a trick of the eye. "I"m working on that," he said.

Galiana halted them next to the other rover. The two machines were parked at the lip of a slope-sided depression in the ice. It was not really steep enough to call a creva.s.se, although it was at least thirty or forty meters deep. From the rover"s cab it was not possible to see all the way into the powdery-blue depths, although Clavain could certainly see the fresh footprints which descended into them. Up on the surface marks like that would have been scoured away by the wind in days or hours, so these prints were very fresh. There were, he observed, two sets-someone heavy and confident and someone lighter, less sure of her footing.

Before they had taken the rover they had made sure there were two suits aboard it. They struggled into them, fiddling with the latches.

"If I"m right," Clavain said, "this kind of precaution isn"t really necessary. Not for avoiding the sickness, anyway. But better safe than sorry."

"Excellent timing," Galiana said, snapping down her helmet and giving it a quarter twist to lock into place. "They"ve just pulled something from your memory, Nevil. There"s a family of single-celled organisms called dinoflagellates, one of which was present in the lab where we found Iverson. Something called pfiesteria piscicida. Normally it"s an ambush predator that attacks fish."

"Could it have been responsible for the madness?"

"It"s at least a strong contender. It has a taste for mammalian tissue as well. If it gets into the human nervous system it produces memory loss, disorientation-as well as a host of physical effects. It couldhave been dispersed as a toxic aerosol; released into the base"s air-system. Someone with access to the lab"s facilities could have turned it from something merely nasty to something deadly, I think."

"We should have pinpointed it, Galiana. Didn"t we swab the air ducts?"

"Yes, but we weren"t looking for something terrestrial. In fact we were excluding terrestrial organisms; only filtering for the basic biochemical building blocks of Diadem life. We just weren"t thinking in criminal terms."

"More fool us," Clavain said.

Suited now, they stepped outside. Clavain began to regret his haste in leaving the base so quickly; at having to make do with these old suits and lacking any means of defense. Wanting something in his hand for moral support, he examined the equipment stowed around the outside of the rover until he found an ice-pick. It would not be much of a weapon, but he felt better for it.

"You won"t need that," Galiana said.

"What if Iverson turns nasty?"

"You still won"t need it."

But he kept it anyway-an ice-pick was an ice-pick, after all-and the two of them walked to the point where the icy ground began to curve over. Clavain examined the wrist of his suit, studying the cryptic and old-fashioned matrix of keypads which controlled the suit"s functions. On a whim he pressed something promising and was gratified when he felt crampons spike from the soles of his boots, anchoring him to the ice.

"Iverson!" he shouted. "Felka!"

But sound carried poorly beyond his helmet, and the ceaseless, whipping wind would have s.n.a.t.c.hed his words away from the creva.s.se. There was nothing for it but to make the difficult trek into the blue depths. He led the way, his heart pounding in his chest, the old suit awkward and top-heavy. He almost lost his footing once or twice, and had to stop to catch his breath once he reached the level bottom of the depression, sweat running into his eyes.

He looked around. The footprints led horizontally for ten or fifteen meters, weaving between fragile, curtainlike formations of opal ice. On some clinical level he acknowledged that the place had a sinister charm-he imagined the wind breathing through those curtains of ice, making ethereal music-but the need to find Felka eclipsed such considerations. He focused only on the low, dark blue hole of a tunnel in the ice ahead of them. The footprints vanished into the tunnel.

"If the b.a.s.t.a.r.d"s taken her..." Clavain said, tightening his grip on the pick. He switched on his helmet light and stooped into the tunnel, Galiana behind him. It was hard going; the tunnel wriggled, rose and descended for many tens of meters, and Clavain was unable to decide whether it was some weird natural feature-carved, perhaps, by a hot sub-glacial river-or whether it had been dug by hand, much more recently. The walls were veined by worm tracks; a marbling like an immense magnification of the human retina. Here and there Clavain saw the dark smudges of worms moving through cracks that were very close to the surface, though he knew it was necessary to stare at them for long seconds before any movement was discernible. He groaned, the stooping becoming painful, and then the tunnel widened out dramatically. He realized that he had emerged in a much larger s.p.a.ce.

It was still underground, although the ceiling glowed with the blue translucence of filtered daylight. The covering of ice could not have been more than a meter or two thick; a thin sh.e.l.l stretched like a dome over tens of meters of yawing nothing. Nearly sheer walls of delicately patterned ice rose up from a level, footprint-dappled floor.

"Ah," said Iverson, who was standing near one wall of the chamber. "You decided to join us."

Clavain felt a stab of relief, seeing that Felka was standing not far from him, next to a piece of equipment Clavain failed to recognize. Felka seemed unharmed. She turned toward him, the peculiar play of light and shade on her helmeted face making her seem older than she was.

"Nevil," he heard Felka say. "h.e.l.lo."

He crossed the ice, fearful that the whole marvelous edifice was about to come crashing down on them all.

"Why did you bring her here, Iverson?" "There"s something I wanted to show her. Something I knew she"d like, even more than the other things." He turned to the smaller figure near him. "Isn"t that right, Felka?"

"Yes."

"And do you like it?"

Her answer was matter of fact, but it was closer to conversation than anything Clavain had ever heard from her lips.

"Yes. I do like it."

Galiana stepped ahead of him and extended a hand to the girl. "Felka? I"m glad you like this place. I like it too. But now it"s time to come back home."

Clavain steeled himself for an argument; some kind of showdown between the two women, but to his immense relief Felka walked casually toward Galiana.

"I"ll take her back to the rover," Galiana said. "I want to make sure she hasn"t had any problems breathing with that old suit on."

A transparent lie, but it would suffice.

Then she spoke to Clavain. It was a tiny thing, almost inconsequential, but she placed it directly in his head.

And he understood what he would have to do.

When they were alone, Clavain said: "You killed him."

"Setterholm?"

"No. You couldn"t have killed Setterholm because you are Setterholm." Clavain looked up, the arc of his helmet light tracing the filamentary patterning until it became too tiny to resolve; blurring into an indistinct haze of detail that curved over into the ceiling itself. It was like admiring a staggeringly ornate fresco.

"Nevil-do me a favor? Check the settings on your suit, in case you"re not getting enough oxygen?"

"There"s nothing wrong with my suit." Clavain smiled, the irony of it all delicious. "In fact, it was the suit which really tipped me off. When you pushed Iverson into the creva.s.se, his helmet came off. That couldn"t have happened unless it wasn"t fixed on properly in the first place-and that couldn"t have happened unless someone had removed it after the two of you left the base."

Setterholm-he was sure the man was Setterholm-snorted derisively, but Clavain continued speaking.

"Here"s my stab at what happened, for what it"s worth. You needed to swap ident.i.ties with Iverson because Iverson had no obvious motive for murdering the others, whereas Setterholm certainly did."

"And I don"t suppose you have any idea what that motive might have been?"

"Give me time; I"ll get there eventually. Let"s just deal with the lone murder first. Changing the electronic records was easy enough-you could even swap Iverson"s picture and medical data for your own-but that was only part of it. You also needed to get Iverson into your clothes and suit, so that we"d a.s.sume the body in the creva.s.se belonged to you, Setterholm. I don"t know exactly how you did it."

"Then perhaps..."

Clavain carried on. "But my guess is you let him catch a dose of the bug you let loose in the main base-pfiesteria, wasn"t it?-then followed him while he went walking outside. You jumped him, knocked him down on the ice and got him out of the suit and into yours. He was probably unconscious by then, I suppose. But then he must have started coming round, or you panicked for another reason.

You jammed the helmet on and pushed him into the creva.s.se. Maybe if all that had happened was his helmet coming off, I wouldn"t have dwelled on it. But he wasn"t dead, and he lived long enough to scratch a message in the ice. I thought it concerned his murderer, but I was wrong. He was trying to tell me who he was. Not Setterholm, but Iverson."

"Nice theory." Setterholm glanced down at a display screen in the back of the machine which squatted next to him. Mounted on a tripod, it resembled a huge pair of binoculars, pointed with a slight elevation toward one wall of the chamber.

"Sometimes, a theory"s all you need. That"s quite a toy you"ve got there, by the way. What is it, somekind of ground-penetrating radar?"

Setterholm brushed aside the question. "If I was him-why would I have done it? Just because I was interested in the ice-worms?"

"It"s simple," Clavain said, hoping the uncertainty he felt was not apparent in his voice. "The others weren"t as convinced as you were of the worms" significance. Only you saw them for what they were."