Stef glanced back at the grinning, somewhat anxious faces of the three left behind. ‘It’s taking an embarrassingly long time to get going,’ she said. ‘I feel like the King of Angleterre in his coronation carriage.’

‘We will be in the dark soon enough,’ the ColU said. ‘But remember, even if the torch were to fail, it is only forty minutes to complete the one-way trip to the far end.’

Now the mouth of the tunnel was all around them, swallowing them up, their speed gradually increasing. The dark was deepening. The movement was utterly smooth, and entirely silent.

Stef felt a frisson of fear. ‘It’s like a roller-coaster ride. Magic Mountain at Disneyland. None of you have the faintest idea what I’m talking about, do you?’

Titus, cradling his torch with his burly body, was suspicious. ‘I don’t understand. We are moving quite rapidly already. And yet there is not a breath of wind.’

‘As I anticipated,’ the ColU said smoothly.

Stef snarled, ‘What now, ColU? I wish you’d be open with us.’

‘I apologise, Colonel Kalinski. There could be no air resistance in here. Otherwise, you see, the friction would slow us; we might pass the midpoint but would not reach the tunnel end, and would slip back, eventually settling at the centre, the lowest point. Human engineering designs based on this idea always imagined a vacuum tunnel.’

Titus took a deep breath. ‘We’re in no vacuum.’

‘I think there is an invisible subtlety to the design. The air we breathe is carried with us – perhaps the tunnel air is held aside. Given time, Stef Kalinski, you and I could no doubt investigate the engineering. Whatever the detail, it must be robust to have survived a billion years …’

The dark was deep now. They didn’t seem to be moving at all, and Stef soon lost track of time. In the light of the torch, Clodia cuddled closer to her father.

Stef, unable to resist it, moved closer to the big Roman too.

Titus said, ‘I am sorry I do not have a hand for you to hold, Stef.’

She clutched his stump of an arm and rested her head on his shoulder. ‘This will do.’

‘It won’t be long,’ the ColU murmured, from the dark. ‘Just forty minutes. Not long.’

They emerged on an icebound plain.

Stef walked a few steps, away from the tunnel mouth and the disgorged cart. She swung her arms, breathing in deeply; the cold stung her mouth, and her breath steamed. ‘This is the far side, all right. Just the way I remember it.’

She looked around. Andromeda still hung huge and looming in a crystal-clear sky; there wasn’t a shred of terminator-weather cloud here. In the crimson galaxy light, the land seemed featureless, flat. But there was a peculiarly symmetrical hillock in the ice a few hundred metres away, like a flattened cone, or a pyramid with multiple flat sides – or like a tremendous jewel, she thought. Could it be artificial? There was no other feature in the landscape to draw her eye.

She walked that way, trying to place her booted feet on ridges in the ice to avoid slipping.

Inevitably Titus called after her. ‘Don’t go too far!’

She snorted. ‘I’m hardly likely to have marauding barbarians leap out at me, legionary.’

‘You might slip and break your brittle old-lady bones. And with my single arm it would be a chore for me to have to carry you back to the cart and haul you home.’

‘I’ll try to be considerate.’

The ColU called, ‘In fact, Colonel Kalinski, would you mind carrying my slate for a closer inspection? And if you could find a way to bring back a sample of that formation …’ With surprising grace on the ice, Clodia jogged out to hand Stef the slate, and a small hammer from their rudimentary tool kit.

As Stef approached the pyramidal structure, she listened to the ColU’s analysis.

‘I can deduce our change in position quite clearly from the shift in the visible stars’ position. Andromeda has shifted too, of course, but that is too large and messy an object to yield a precise reading …’

The closer she got, the less like a geological formation the pyramid seemed. It was too precise, too sharply defined for that. She supposed there might be a comparison with something like a quartz crystal. But she had an instinct that there was biology at work here, something more than mere physics and chemistry. She took panoramic and close-up images. The pyramid looked spectacular and utterly alien, sitting as it was beneath a sky full of galaxy. Then she bent to chip off a sample from one gleaming, perfect edge.

Titus called, ‘How far have we travelled then, glass demon?’

‘Not very far at all, Titus Valerius. Only a hundred kilometres – just a little more. That’s perhaps sixty Roman miles. Not very far – but that means we were never very deep under the surface. Two hundred metres at the lowest point, perhaps.’

With her sample of what felt like water ice tucked into an outer pocket, Stef headed carefully back to the group.

‘Not very far, as you say, demon. But we know this tunnel is not the only one of its kind in the planet.’

‘Quite so, legionary. There will be many such links, perhaps a whole network, perhaps of varying lengths.’

‘Yes. And a way for us to go on, deeper into the dark. There must be another entrance close by – all we need do is find it. And then—’

‘And then we can proceed in comparative comfort, if we’re lucky, all the way to the antistellar,’ said the ColU. ‘For that central locus must be a key node of any transport network.’

Stef had got back to the cart, within which the ColU sat, bundled against the cold. ‘You want me to put some of this sample in your little analysis lab?’

‘Yes, please, Stef Kalinski. Titus Valerius, let us consider. If this length of tunnel is typical, at sixty miles or so, and if we have a journey of less than six thousand Roman miles to complete to the antistellar—’

‘We’ll need a hundred hops. And if each hop takes us two-thirds of an hour, as you said, that will take, umm …’

‘Sixty, seventy hours,’ Stef said. ‘I always was good at mental arithmetic. Even allowing for stops, and for hauling the cart between terminals, that’s only a few days.’

‘It may be hard work,’ Titus said. ‘But we will not freeze to death, or starve, or die of thirst on the way.’ He nodded. ‘Excellent! But you know, Stef, I, Titus Valerius, anticipated that we would find some such fast road as this.’

‘You did? How?’

‘Because if not, we would have encountered Ari Guthfrithson and the Inca woman walking back the other way. Would we not? For if we could never have mastered this world of ice on foot, and I suspect that is true, they could surely not. Clever fellow, aren’t I, for a one-winged legionary? Now then – Clodia, come with me. We will do a little scouting before we return. Let’s see if we can find the terminal of the next link, somewhere in the direction of the antistellar …’ He glanced up at the sky, taking a bearing from Andromeda. ‘That way. Come now! And you, Stef Kalinski, you and your old-lady bones stay put in this cart.’

‘With pleasure, legionary.’

As they walked away, she heard father and daughter laughing.

‘It’s good to hear them happy,’ Stef said. ‘Suddenly a journey that did look impossible has become achievable.’

‘You too should be happy,’ the ColU whispered.

‘I should?’

‘For the discovery you have just made.’

‘What discovery? The pyramid?’

‘It’s no pyramid, Stef Kalinski. It’s nothing artificial, and nor is it a merely physical phenomenon, as I’m sure you guessed. It is life, Stef Kalinski. Life. An ambassador, perhaps, from a colder world than this …’

As they sat huddled together in the cart, the ColU spoke of Titan, moon of Saturn.

Titan was a mere moon, a small world subsidiary to a giant, but a world nevertheless – and a very cold one. Its rocky core was overlaid by a thick shell of water, a super-cold ocean contained by a crust of ice as hard as basalt was on Earth. And over that was a thick atmosphere, mostly of nitrogen, but with traces of organics, methane, ethane …