Eilidh just returned her look. ‘But we are speaking of the Hatches. The first Hatch of all was found on Mercury, in the kernel field.’

‘As it was for us,’ Stef said.

Eilidh raised her eyebrows. ‘On a different Mercury too? We do have much to discuss. Of course the Hatch was opened; of course there were attempts to pass through  … None of those who entered, unwilling slaves, bold soldiers, curious philosophers, ever returned.’

‘Perhaps they are still in transit.’

‘In transit?’

‘Our Mercury Hatch as connected to one on Per Ardua. Umm, which is a world of Proxima Centauri. Which is—’

‘The nearest star, in the Centaur’s Hoof. For us, it has been given the same name. Proxima.’ She smiled, a little sourly. ‘So there are Romans in your country too.’

‘Were. Long story. Look, it’s only four years as light travels between Mercury and Proxima. So it’s possible to go there and step back with only eight years elapsing.’

Eilidh frowned as she puzzled all that out; Stef had no idea how much understanding of such basic physics they shared.

‘The point is,’ Stef said, ‘maybe your Hatch on your Mercury was hooked up to somewhere else. Somewhere much further away.’ There was no reason why that shouldn’t be true, she realised. They knew so little, despite the decades that had passed since her own first brush with all this strangeness. ‘Your travellers may have arrived alive and well, but just haven’t had time to step back home yet. Maybe they are still travelling, oblivious.’

‘It’s possible. Oddly there is a soldiers’ legend along those lines. Perhaps the travellers have gone, not to Proxima, the nearest star, but to Ultima, the furthest star of all.’

Stef frowned. What could that mean? The furthest star, in an expanding universe full of galaxies and clusters of galaxies …

‘But, though we have not walked through the Hatches to Proxima and its worlds, we have journeyed there in ships – ships like the Malleus Jesu, orbiting high above. When we got there, on the third planet from the star –’

Per Ardua.

‘– we found a kernel field, not unlike that on Mercury – by then we had learned how to search for such things – and we found a Hatch, and we found instructions on how to construct a fresh one. Just as I have described.’

‘Instructions. Of what kind?’

‘Enigmatic. Graphic, but enigmatic. Enough for us to work out the rest, after—’

‘Another blood toll.’ Stef remembered the builders, natives of Per Ardua – her Per Ardua. She had seen little of them, but she knew Yuri remembered them with affection from his early, near-solitary years on the planet. ‘These graphic instructions – was there any sign of the natives who created them?’

‘None. So I’m told. Not a trace save these odd diagrams, and even they were lodged inside a Hatch.’ She eyed Stef. ‘It was another scrap that doesn’t fit, another fragment of a lost history. Like you and your companions. What do you think?’

A scrap like her own unexpected sister in the Hatch on Mercury, Stef thought. The first reality tweak of all. She shrugged. ‘I don’t know what to think.’

‘Well, keep trying. And now – look down.’

The cetus was now sailing serenely over mountains.

The sun of this world was not high, it might have been an early afternoon at a temperate latitude on Earth, and shadows pooled in the valleys that separated the peaks. The second sun was in the sky too and cast a fainter double shadow. Ice striped the taller peaks, and rivers flowed through the valleys like bands of steel. And, save for the shadow cast by the cetus itself, Stef could see nothing moving down there, no people, no animals, not so much as a thread of smoke.

But everywhere she looked, Stef saw artifice. Every mountain seemed to have been shaped, regularised as a pyramid or a tetrahedron. The valleys looked as if they had been shaped, too, straightened. Some of the peaks were connected by tremendous bridges of stone. Many of the mountain walls were terraced, so that it looked as if giant staircases climbed their flanks, while others had huge vertical structures fixed to their faces, almost like the flying buttresses of medieval cathedrals, or were deeply inscribed with gullies and channels.

Eilidh was watching her. ‘Tell me what you see.’

‘It’s like a simulation.’

‘A what?’

‘Sorry. Like a model. A mock-up of a mountain range. It doesn’t look real.’

‘Yet it is real. This planet is laced by mountain ranges; it is, or at least was, very active. And all the ranges have been shaped and reshaped by hands unseen, just as you see here. All as far as we have visited and studied. There’s much you can’t see from the surface. We burrowed into one mountain, sounded out others. The mountains are hollowed, strengthened within by huge remnant pillars of rock. They have been transformed into immense granite fortresses, or so it seems. For the Roman military engineers, who eat and breathe fortifications, this is Elysium, as you can imagine.’

‘We noticed this the minute we stepped out of the Hatch,’ Stef said, wondering. ‘I never dreamed the whole world was like this. But – who built all this? And where are they now?’

‘That’s the puzzle. These vast mountain-fortresses are all pristine, save for some evidence of erosion and rock fall – natural breakdowns. There’s no evidence they were ever inhabited, let alone fought over. Meanwhile, across the planet, we have found no trace of life more complex than those orange chimney stacks of bugs you see piled up on the plains. Nothing moved here, not until the legionaries arrived, and they don’t move much either. Ha! I do have a theory, for what it’s worth. I may be limited as a druidh but I’ve seen as much of this world as anybody.’

‘Tell me.’

‘The farside, the dark side, is – damaged. I’ve seen vast craters, their rims protruding above the ice. And there is a very odd range of mountains running virtually north to south down the rim of one of the continents there, buried though it is under the ice.’

‘Like the Andes.’

‘The what?’

‘A mountain range in, umm, Valhalla Inferior, I think you call it.’

‘Like that – yes. Now, these mountains had been modified, but not as fortresses. We saw evidence of vast installations, like cannon muzzles, all along the western faces of the mountains. My colleagues, especially the Romans, thought these must be weapons, but they didn’t look like very effective weapons to me. The only purpose I could think of …’

‘Yes?’

‘Perhaps these were, not weapons, engines. Rockets intended to fire together, powered by kernels presumably, blasting all along this great seam along the belly of the planet—’

‘My God. You think they were trying to spin up the planet?’

‘It’s possible. Maybe there was some great project to make this world more hospitable. The approaches to the second sun, you know, do make life difficult here, for the native life as for the Roman colonists.’

The ColU had worked out that this was a double-star system in which both partners were red dwarfs – small, miserly stars, like Proxima, so small and dim they hadn’t even been detected from Earth. The ColU had said the nearest such system to Earth must be at least seven, eight, nine light years out.

‘Of course,’ Eilidh said, ‘most of this world’s life, like every living world, is comprised of bugs that inhabit the deep rocks, miles deep, feeding off seeps of water and heat and minerals. We found them here when we were running deep mining trials – as one always finds them, on every world. They won’t care if there is one or two suns in the sky, or more. So long as the world itself lasts, they will too.’

‘I take it the great spin-up never happened.’

‘It appears there was a war to stop it. Evidently not everybody agreed with the visionary engineers behind the scheme. The big spin-mountain engines were attacked – we have seen the damage.’