I allowed myself three full breaths, lying there on the cold ground, as I checked I was still in one piece.

Then I stood and pulled off my visor. The air was breathable, but thick with the smell of burning, and of sulphur. But the ground quivered under my feet, over and over. I wasn't too troubled by that-until I reminded myself that planets were supposed to be stable.

Lian was standing there, her suit glowing softly. "Good landing, sir," she said.

I nodded, glad she was safe, but irritated; if she'd followed orders she wouldn't have been here at all. I turned away from her, a deliberate snub that was enough admonishment for now.

The sky was deep. Beyond clouds of ash, sunsats swam. Past them I glimpsed the red pinprick of the true sun, and the wraith-like galaxy disc beyond.

Behind me the valley skirted the base of Mount Perfect, neatly separating it from more broken ground beyond. The landscape was dark green, its contours coated by forest, and clear streams bubbled into a river that ran down the valley's center. A single, elegant bridge spanned the valley, reaching toward the old Conurbation. Further upstream I saw what looked like a logging plant, giant pieces of yellow-colored equipment standing idle amid huge piles of sawn trees. Idyllic, if you liked that kind of thing, which I didn't.

On this side of the valley, the village was just a huddle of huts-some of them made from wood-clustered on the lower slopes of the mountain. Bigger buildings might have been a school, a medical center. There were a couple of battered ground transports. Beyond, I glimpsed the rectangular shapes of fields-apparently plowed, not a glimmer of replicator technology in sight, mostly covered in ash.

People were standing, watching me, gray as the ground under their feet. Men, women, children, infants in arms, old folk, people in little clusters. There were maybe thirty of them.

Lian stood close to me. "Sir, I don't understand."

"These are families, " I murmured. "You'll pick it up." "Dark matter." The new voice was harsh, damaged by smoke; I turned, startled.

A man was limping toward me. About my height and age but a lot leaner, he was wearing a tattered Navy coverall, and he was using an improvised crutch to hobble over the rocky ground, favoring what looked like a broken leg. His face and hair were gray with the ash.

I said, "You're the academician."

"Yes, I'm Tilo."

"We're here to get you out."

He barked a laugh. "Sure you are. Listen to me. Dark matter. That's why the Xeelee are here. It may have nothing to do with us at all. Things are going to happen fast. If I don't get out of here-whatever happens, just remember that one thing..."

Now a woman hurried toward me. One of the locals, she was wearing a simple shift of woven cloth, and leather sandals on her feet; she looked maybe forty, strong, tired. An antique translator box hovered at her shoulder. "My name is Doel," she said. "We saw you fall-"

"Are you in charge here?"

"I-" She smiled. "Yes, if you like. Will you help us get out of here?"

She didn't look, or talk, or act, like any Expansion citizen I had ever met. Things truly had drifted here. "You are in the wrong place." I was annoyed how prissy I sounded. I pointed to the Conurbation, on the other side of the valley. "That's where you're supposed to be."

"I'm sorry," she said, bemused. "We've lived here since my grandfather's time. We didn't like it, over in Blessed. We came here to live a different way. No replicators. Crops we grow ourselves.

Clothes we make-"

"Mothers and fathers and grandfathers," Tilo cackled. "What do you think of that, Lieutenant?"

"Academician, why are you here?"

He shrugged. "I came to study the mountain, as an exemplar of the planet's geology. I accepted the hospitality of these people. That's all. I got to like them, despite their-alien culture."

"But you left your equipment behind," I snapped. "You don't have comms implants. You didn't even bring your mnemonic fluid, did you?"

"I brought my pickup beacon," he said smugly.

"Lethe, I don't have time for this." I turned to Doel. "Look-if you can get yourself across the valley, to where that transport is, you'll be taken out with the rest."

"But I don't think there will be time-"

I ignored her. "Academician, can you walk?"

Tilo laughed. "No. And you can't hear the mountain."

That was when Mount Perfect exploded.

Tilo told me later that, if I'd known where and how to look, I could have seen the north side of the mountain bulging out. The defect had been growing visibly, at a meter a day. Well, I didn't notice that.

Thanks to some trick of acoustics, I didn't even hear the eruption-though it was slightly heard by other Navy teams working hundreds of kilometers away.

But the aftermath was clear enough. With Lian and Doel, and with Academician Tilo limping after us, I ran to the crest of a ridge to see down the length of the valley.

A sharp earthquake had caused the mountain's swollen flank to shear and fall away. As we watched, a billion tons of rock slid into the valley in a monstrous landslide. Already a huge gray thunderhead of smoke and ash was rearing up to the murky sky.

But that was only the start, for the removal of all that weight was like opening a pressurized can. The mountain erupted-not upward, but sideways, like the blast of an immense weapon, a volley of superheated gas and pulverized rock. It quickly overtook the landslide, and I saw it roll over trees-imports from distant Earth, great vegetable sentinels centuries old, flattened like straws.

I was stupefied by the scale of it all.

Now, from out of the ripped-open side of the mountain, a chthonic blood oozed, yellow-gray, viscous, steaming hot. It began to flow down the mountainside, spilling into rain-cut valleys. "That's a lahar," Tilo murmured. "Mud. I've learned a lot of esoteric geology here, Lieutenant....

The heat is melting the permafrost-these mountains were snow-covered two weeks ago; did you know that?-making up a thick mixture of volcanic debris and meltwater."

"So it's just mud," said Lian uncertainly.

"You aren't an earthworm, are you, marine?"

"Look at the logging camp," Doel murmured, pointing.

Already the mud had overwhelmed the heavy equipment, big yellow tractors and huge cables and chains used for hauling logs, crumpling it all like paper. Piles of sawn logs were spilled, immense wooden beams shoved downstream effortlessly. The mud, gray and yellow, was steaming, oddly like curdled milk.

For the first time I began to consider the contingency that we might not get out of here. In which case my primary mission was to preserve Tilo's data.

I quickly used my suit to establish an uplink. We were able to access Tilo's records, stored in cranial implants, and fire them up to the Spline. But in case it didn't work- "Tell me about dark matter," I said. "Quickly."

Tilo pointed up at the sky. "That star-the natural sun, the dwarf-shouldn't exist."

"What?"

"It's too small. It has only around a twentieth Earth's sun's mass. It should be a planet: a brown dwarf, like a big, fat Jovian. It shouldn't burn-not yet. You understand that stars form from the interstellar medium-gas and dust, originally just Big-Bang hydrogen and helium. But stars bake heavy elements, like metals, in their interiors, and eject them back into the medium when the stars die. So as time goes on, the medium is increasingly polluted."

Impatiently I snapped, "And the point-"

"The point is that an increase in impurities in the medium lowers the critical mass needed for a star to be big enough to burn hydrogen. Smaller stars start lighting up. Lieutenant, that star shouldn't be shining.

Not in this era, not for trillions of years yet; the interstellar medium is too clean....You know, it's so small that its surface temperature isn't thousands of degrees, like Earth's sun, but the freezing point of water.

That is a star with ice clouds in its atmosphere. There may even be liquid water on its surface."

I looked up, wishing I could see the frozen star better. Despite the urgency of the moment I shivered, confronted by strangeness, a vision from trillions of years downstream.

Tilo said bookishly, "What does this mean? It means that out here in the halo, something, some agent, is making the interstellar medium dirtier than it ought to be. The only way to do that is by making the stars grow old." He waved a hand at the cluttered sky. "And if you look, you can see it all over this part of the halo; the H-R diagrams are impossibly skewed...."

I shook my head; I was far out of my depth. What could make a star grow old too fast?...Oh. "Dark matter?"

"The matter we're made of-baryonic matter, protons and neutrons and the rest-is only about a tenth the universe's total. The rest is dark matter: subject only to gravity and the weak nuclear force, impervious to electromagnetism. Dark matter came out of the Big Bang, just like the baryonic stuff. As our galaxy coalesced the dark matter was squeezed out of the disc.... But this is the domain of dark matter, Lieu tenant. Out here in the halo."

"And this stuff can affect the ageing of stars."

"Yes. A dark matter concentration in the core of a star can change temperatures, and so affect fusion rates."

"You said an agent was ageing the stars." I thought that over. "You make it sound intentional."

He was cautious now, an academician who didn't want to commit himself. "The stellar disruption appears non-random."

Through all the jargon, I tried to figure out what this meant. "Something is using the dark matter?...Or are there life forms in the dark matter? And what does that have to do with the Xeelee, and the problems here on Shade?"

His face twisted. "I haven't figured out the links yet. There's a lot of history. I need my data desk,"he said plaintively.

I pulled my chin, thinking of the bigger picture. "Academician, you're on an assignment for the admiral. Do you think you're finding what he wants to hear?"

He eyed me carefully. "The admiral is part of a-grouping-within the Navy that is keen to go to war, even to provoke conflict. Some call them extremists. Kard's actions have to be seen in this light."

Actually, I'd heard such rumors, but I stiffened. "He's my commanding officer. That's all that matters."

Tilo sighed. "Mine too, in a sense. But-"

"Lethe," Lian said suddenly. "Sorry, sir. But that mud is moving fast."

So it was, I saw.

The flow was shaped by the morphology of the valley, but its front was tens of meters high, and it would soon reach the village. And I could see that the gush out of the mountain's side showed no signs of abating. The mud was evidently powerfully corrosive; the land's green coat was ripped away to reveal bare rock, and the mud was visibly eating away at the walls of the valley itself. I saw soil and rock collapse into the flow. Overlaying the crack of tree trunks and the clatter of rock, there was a noise like the feet of a vast running crowd, and a sour, sulphurous smell hit me.

"I can't believe how fast this stuff is rising," I said to Tilo. "The volume you'd need to fill up a valley like this-"

"You and I are used to spacecraft, Lieutenant. The dimensions of human engineering. Planets are big.

And when they turn against you-"

"We can still get you out of here. With these suits we can get you over that bridge and to the transport-"

"What about the villagers?"

I was aware of the woman, Doel, standing beside me silently. Which, of course, made me feel worse than if she'd yelled and begged.

"We have mixed objectives," I said weakly.

There was a scream. We looked down the ridge and saw that the mud had already reached the lower buildings. A young couple with a kid were standing on the roof of a low hut, about to get cut off.

Lian said, "Sir?"

I waited one more heartbeat, as the mud began to wash over that hut's porch.

"Lethe, Lethe." I ran down the ridge until I hit the mud.

Even with the suit's augmentation the mud was difficult stuff to wade through-lukewarm, and with a consistency like wet cement. The stench was bad enough for me to pull my visor over my mouth. On the mud's surface were dead fish that must have jumped out of the river to escape the heat. There was a lot of debris in the flow, from dust to pebbles to small boulders: no wonder it was so abrasive.

By the time I reached the little cottage I was already tiring badly.

The woman was bigger, obviously stronger than the man. I had her take her infant over her head, while I slung the man over my shoulder. With me leading, and the woman grabbing onto my belt, we waded back toward the higher ground.

All this time the mud rose relentlessly, filling up the valley as if it had been dammed, and every step sapped my energy.

Lian and Doel helped us out of the dirt. I threw myself to the ground, breathing hard. The young woman's legs had been battered by rocks in the flow; she had lost one sandal, and her trouser legs had been stripped away.

"We're already cut off from the bridge," Lian said softly.

I forced myself to my feet. I picked out a building-not the largest, not the highest, but a good compromise. "That one. We'll get them onto the roof. I'll call for another pickup."

"Sir, but what if the mud keeps rising?"

"Then we'll think of something else," I snapped. "Let's get on with it."

She was crestfallen, but she ran to help as Doel improvised a ladder from a trellis fence. My first priority was to get Tilo safely lodged on the roof. Then I began to shepherd the locals up there. But we couldn't reach all of them before the relentless rise of the mud left us all ankle-deep.

People began to clamber up to whatever high ground they could find-verandas, piles of boxes, the ground transports, even rocks. Soon maybe a dozen were stranded, scattered around a landscape turning gray and slick.

I waded in once more, heading toward two young women who crouched on the roof of a small building, like a storage hut. But before I got there the hut, undermined, suddenly collapsed, pitching the women into the flow. One of them bobbed up and was pushed against a stand of trees, where she got stuck, apparently unharmed. But the other tipped over and slipped out of sight.

I reached the woman in the trees and pulled her out. The other was gone.

I hauled myself back onto the roof for a break. All around us the mud flowed, a foul-smelling gray river, littered with bits of wood and rock.

I'd never met that woman. It was as if I had become part of this little community, all against my will, as we huddled together on that crudely built wooden roof. Not to mention the fact that I now wouldn't be able to fulfill my orders completely.

The loss was visceral. I prepared to plunge back into the flow.

Tilo grabbed my arm. "No. You are exhausted. Anyhow you have a call to make, remember?"