Not far from the camp the trail petered out. Yuri guessed the builders had realised they weren’t being followed, and had slowed down, taken more care with the precious fragments they were carting home. Lacking any better clues Yuri just kept walking the way he’d been heading, taking a line of sight between the camp and features of the lake: a swampy area by the shore, a cloud of kites flapping in very birdlike flocks over the water.

And as he approached the lake he saw he was heading straight for the big heap of dead stems, the midden he’d been taking the stuff from in the first place.

He came to a bluff, a tilted slab of stratified stone taller than he was that offered a little shade. He took a break from the sun, a swig of water from one of his bottles.

Here in the shade the ground was quite bare, he saw, the rock faces clean of the native lichen. He kept forgetting that here on Per Ardua the shadows never shifted; this little scrap of ground was in permanent shadow, the only light coming from reflection from the ground, so little could ever grow here. Further north, he thought, there must be places where Proxima light never reached, where the ground was forever frozen, the snow never melted. He wondered if he’d ever go that far. Maybe not, if he was stuck by this lake the whole of his life.

He walked on, coming to the lake shore just to the west of the midden. From here the way the land rose gradually to the north, beyond the lake, was very obvious – and getting more so, if the ColU was right about the geology and the changes.

The midden itself was a heap of stems, a rough arc facing the water. He could see similar structures further to the west, all along the lake’s southern shore. But he couldn’t remember seeing these before. Were they new, had they been built up? They looked almost like pieces of an incomplete dam, he thought now.

Before him the lake itself was shallow, nearly choked with banks of the reed-like stems. A flock of kites drifted on the lake. They seemed to feed on the stems in the water; he’d seen them plucking stems and tucking them into their bodies, especially their densely woven cores. But sometimes they would break the stems, and finer appendages on the kites, like drinking straws, would be dipped in to extract the sticky marrow within. He was too far away to see the details of how they did this, how creatures like bundles of sticks in brown paper could manage such fine operations. Then they lifted suddenly into the air, flapping, splashing. They were very birdlike in their movements on the water, like gaunt pelicans maybe, an illusion broken when they flew up and you could see those twin sets of spinning vanes, like some kid’s rubberband toy of a helicopter.

And he spotted movement on the big midden.

He stepped back, trying to stay inconspicuous.

It was a party of builders, tripods silhouetted against the sky – seven, eight, nine of them, burdened with dead stems. Surely the party he’d been following. He saw they’d piled up the bundle they’d taken from the camp on the top of the midden, and with some care were threading the individual stems back into the structure, like reassembling a haystack one straw at a time. This obviously mattered to them, to go to all the trouble of retrieving the stuff, and to handle it so carefully.

Now another party of builders approached the midden. Just three of them, they moved together, in a fluid triangle of which one vertex moved at a time, so the formation swivelled across the muddy ground. They moved like this because they were carrying something, he saw, handing it off gracefully one to the other as they moved. It looked like just another bundle of stems to Yuri, until they started to climb up the slope of the midden.

Then he saw that the bundle was actually a body, another tripod-shaped builder, inert, its component stems clattering loosely as the party laboured up the mound.

Near the top they laid down their burden. With swift, precise movements they began to disarticulate it, separating the stems at the joints. Moving slowly, hoping not to be seen, Yuri dug his small telescope out of his pocket for a closer look. They were using knives, just chips of stone, jet black, basalt from the Lip maybe, gripped in combinations of fine stems like skeletal hands. With these stone knives they cut through the marrow blobs connecting the joints of the corpse. When they were done they began to lay out the disconnected stems across the surface of the midden, setting them down with great care, in a pattern Yuri could not see, and no doubt would not have understood.

They stood over the remains, the three of them in a neat row, utterly motionless. It was a funeral party, he realised.

And then, as one, they broke away from each other, spinning off in diverse directions. One of them headed west. Yuri followed it, at random.

As he walked, he got out his slate and murmured quick notes. ‘They plan. They work together. They have tools, knives at least. They honour their dead. No wonder they raided us. I’ve been robbing their cemetery . . .’

A little way around the curve of the lake shore, the builder he was following approached a thick bed of reed-like stems, just away from the water’s edge. In the background there was a magnificent row of stromatolites, as big as any Yuri had found elsewhere, tremendous flat-topped mounds whose surfaces shone like bronze. Yuri saw the builder was heading for a kind of dome assembled from stems that reminded Yuri of a bird’s nest, big, upside down – not that he’d seen a bird’s nest since his parents committed him to cryo. The colonists had always called these things ‘shelters’, but Yuri had no idea if that was their true purpose. The builder pushed its way inside this structure with a rustle.

Yuri crouched down and waited.

After a few minutes the builder emerged again, and went spinning off into one of the stem beds near the water.

Overwhelmed with curiosity, Yuri crept forward to the shelter. Close to, the structure looked densely woven, seamless. But he remembered where the builder had entered it – indeed there were trails in the mud, overlapping circular scrapings where it had passed. The builder had gone in through a soft place in the dome, a slit he could shove his hand inside.

Yuri got down on his hands and knees and pushed forward into darkness only relieved a little by the daylight seeping in behind him.

Once inside, he could see nothing. He pulled his pack over his shoulder and rifled through it in the dark. He never carried a torch; you didn’t need a torch, in the unending afternoon of Proxima. But he dug out his slate, tapped it a couple of times to bring up a bright glowing display. He turned it, shining the light into the interior.

He saw more builders: little ones, stationary, like models, or toys. They stood amid mounds of stems, heaps of stone flakes, other objects he couldn’t identify, just shapes in the uncertain light.

He set down the slate and picked up the smallest builder. It was only ten centimetres tall, maybe, and it was simple, especially in its internal structure, the mesh core. It was like a stool for a child. He turned it over and over.

One stem suddenly shot out of the axis of the little builder’s central core, broader, flatter than usual, like a leaf, darker. And, with a rustle, an eye opened, right in the middle of the leaf, an eye that might have been human, with white and an iris and even a pupil, staring right back at him.

‘Shit!’ Suddenly the little builder began to squirm in his hands. It was like he was wrestling with an animated bundle of sticks, a wooden puppet come to life, with that eerie eye glaring at him. ‘Shit, shit!’ He dropped the builder, knocking aside his slate in the process.

There was hardly any light now, and he could hear the little builder and its fellows running around in the dark with a chattering rustle of stems. Suddenly, here in the dark with these strange creatures, he had a deep, almost phobic reaction; he had to get out of here. He felt for his slate and his pack and backed out into the bright air.