‘It should be safe enough,’ the ColU said.

Mardina turned on it. ‘What? What? Are you serious? How can you possibly say that?’

The ColU stayed calm. ‘Evidently we are dealing with some distortion of space and time. There may be some kind of machinery in the mouth of the pit – exotic matter of some kind, perhaps, or a tremendous gravitational engine. But the builders passed safely through the opening. If there are any hazards, tidal effects perhaps, they are evidently gentle enough—’

‘Give me that ladder.’ Beth took it from Tollemache and dropped it into the hole. It passed through the hatch opening as easily as the builders had, Yuri noticed. Then she began to clamber down.

Tollemache watched her admiringly. He murmured to Yuri, ‘I will never know how something as piss-poor as you, ice boy, produced something as lush as that.’

‘Fuck you,’ Mardina said simply, her voice taut with anxiety.

On the pit floor, Beth stepped back from the ladder, looked up, spread her arms, turned around. The builders spun around her, their stem limbs making soft scraping noises on the sheer surfaces. She called, ‘Look at me. Safe as I ever was. Are you coming down, or not?’

Mardina remained cautious. She made her daughter climb back out first, just to ensure that it was possible, that they weren’t dealing with some kind of one-way trap.

Then Tollemache was the first to follow Beth back into the hole. ‘Me next. I’m not missing out on this.’ He made sure his camera pack followed his own progress down the ladder. ‘Just like Dexter Cole. One small step for a man, like he said. Or was that Cao Xi on Mars?’

Beth blew a raspberry into his camera.

Mardina and Yuri exchanged glances. ‘I’ll go,’ said Yuri. ‘You wait.’

‘No way. I’m not letting Beth out of reach.’

‘Well, I’m not letting the two of you go anywhere without me.’

‘We can’t both go. Somebody ought to stay up top, in case—’

The ColU said gravely, ‘I can call for help if there is trouble. I can even block the lid if it descends, perhaps. This is a human adventure, Lieutenant Jones, Yuri Eden. Perhaps in some ways it is why humans have come to this world.’

Mardina frowned. ‘What does that mean? Oh, the hell with it.’ She went down the ladder.

Yuri patted the ColU’s battered hull. ‘See you later, buddy.’

As he climbed down the ladder in his turn, he felt nothing as he entered the pit, passing from the world of the real into the realm of the impossible. No tugging, no tide effects, no shift of perception.

At the bottom, he was just in some smooth-walled hole in the ground, with the three others. They looked at each other, then stared around. There was plenty of room for them all, and the spinning, darting builders. Up above Yuri saw the cloudy sky of Per Ardua’s substellar point, with a fringe of foliage, and the ColU’s sensor pod held out over them all, quietly watching, recording.

Mardina passed her hands over the wall surface. The glowing light shadowed the bones within the flesh. ‘It feels slick, frictionless.’

Beth was inspecting the tapestry on the wall. ‘This looks like it’s stuck on with stem marrow.’

Yuri, Mardina and Beth stood together before the object. Maybe a half-metre square, it was made of some kind of fine-woven stem-bark cloth held open by a frame of four neat stems, which looked the right size once to have been builder limbs. It bore an image of a disc, washes of brown and blue-grey, hanging before a watery blue sky, all marked in some kind of pigment. If you looked more closely there was a great deal of detail, a furry fringe at the perimeter of the circle, a dense grey navel at the very centre, and fine blue threads that crisscrossed the disc, linking at dense nodes. The threads reminded Yuri of a chart of great-circle airline routes.

‘It is a map, isn’t it?’ Beth asked. ‘Just as it looks.’

Yuri shrugged. ‘What else can it be?’

‘A map of the whole world,’ Mardina said, wondering. ‘Just like we’d draw. The world as seen from space, from Proxima. There’s the substellar point at the centre. There’s the fringe forest. Look at that big bay cutting into the main continent – in the west? Builders made this.’

Yuri hesitated. ‘I’ve never seen a builder make a map. But they know their way around the landscape, we know that.’

Beth seemed defensive of the builders. ‘The ColU seems to think they built this whole place.’

‘Hmm,’ Mardina said. ‘But this map’s a lot cruder. And it’s just stuck on the wall.’

Yuri said, ‘So the builders once made high-tech installations, like this, with radioactivity and heavy elements, and other shit. Then, later, all they could make was a map to stick on the wall. And now all they can do is spin around keeping the mud off – if we let them.’

Beth looked troubled. ‘What does it all mean, Dad?’

‘Damned if I know, sweetie.’

‘I wonder how old it is,’ Mardina said. ‘The map. Maybe we could tell if it’s drawn accurately enough, from continental drift, or something.’

‘That takes millions of years to make a difference. This can’t be that old . . . can it?’

Mardina shrugged. ‘All the ColU could find of some kind of advanced industrial installation outside was a few scrapings of polluted dirt. It would take a fusion plant, say, a long time to break down that far.’

Beth traced the mesh of lines that overlaid the map of the world. ‘What are these?’

‘They look like canals,’ Yuri said. ‘They make Per Ardua look like Mars was supposed to be.’

Neither of them knew what he was talking about. Before their time.

‘The builders don’t do canals,’ Mardina said.

‘Not that we’ve seen. But they do a lot of water management. They move lakes.’

‘Nothing on this scale. Why, some of these canals cross the heart of the continent – they have to be channelled through bedrock. If they had ever existed, they’d leave a trace, even if ice ages had come and gone across the face of this world. In the Ad Astra, we did make some surveys from orbit. We’d have seen canals. And on the ground, we walked a long way. We’d have noticed the things, we’d have had to cross them.’

‘Then the map’s wrong.’

‘Or maybe the map’s right,’ said Beth. ‘And the world is wrong.’

Yuri stared at her. ‘That makes no sense. Does it?’

‘There’s something else you’re missing,’ Tollemache called.

They looked over. The three builders had shimmied up the frictionless walls and were inserting themselves into the three sets of grooved ‘key’ beds in the hatch in the wall.

‘The second hatch,’ Yuri said. ‘Shit. I forgot. And these builders are about to open it. Here’s another of those choice points. Do we go on, or go back?’

Mardina said tensely, ‘I was trained up as an astronaut. And one thing that was driven home to us was that you don’t go opening hatches just because they’re there.’

‘Well, we’re not in space, Mom,’ Beth said.

The three builders were settling into their positions.

‘Last chance to run,’ Tollemache said.

None of them moved. The decision made itself. Mardina grabbed Yuri’s and Beth’s hands. Tollemache seemed to brace himself.

With a soft sigh, the hatch in the wall swung away, taking the spread-eagled builders with it.

CHAPTER 58

The chamber beyond the wall hatch was almost an anticlimax. It seemed to be a copy of the room they were leaving, another cylinder a few metres across, though with a closed roof just as seamlessly joined to the walls as the floor, and similarly glowing with a sourceless mother-of-pearl light. But there was yet another hatch on the far side of the room, once again engraved with builder-body lock grooves.

The three builders leapt through the second hatch and spun around the floor, joyful once more, as if glad to be back here.