It was another ellipse, tilted like Andromeda in the sky – but this one, much longer than it was wide, was cut into the ground. And as Stef approached the cut she saw that in fact she was looking into a circular tube, a cylinder – no, a tunnel, it was big enough to be called that, several metres in diameter, that slid into the ground at a shallow angle, making this elliptical cross-section where it met the flat ground surface.

The ColU had his bearer, Chu, walk around this formation, studying it closely.

But Titus warned them all sternly not to step into the tunnel, onto the smooth curved interior. ‘We were wandering around at random, hoping to find a convenient river or some such to carry us further on our way … Then we found a kind of way marker. Solid granite, and barely eroded.’

‘We are all but beyond the terminator weather here,’ the ColU said. ‘Weathering, erosion, will be slow. The marker, like this structure, could be extremely old.’

‘Well, the marker had a distinctive arrow, you couldn’t mistake its meaning. Which led us straight to this.’

‘Remarkable,’ the ColU said. ‘Remarkable. And for us to have happened on such a structure so close to where we crossed the terminator – it cannot be chance; the cold side of this world must be laced with such constructs.’

‘I don’t understand,’ said Stef. She walked closer to the ellipse lip. ‘I see a tunnel.’ She glanced back at their mountain for reference. ‘Pointing pretty much south-east – that is, away from the substellar—’

‘And directly towards the antistellar,’ the ColU said.

‘A tunnel sloping down at a pretty shallow angle.’ She took Clodia’s torch and held it up. The tunnel continued dead straight, into the ground, beyond the glow cast by the flickering torch. ‘Some kind of transport system?’

Titus grinned. ‘You philosophers haven’t spotted the most interesting thing about it. I told you to stay off the surface. Why? Because it is perfectly slippery – less friction than the smoothest ice, I would say. Though I can tell you it is no colder than the rest of the world – I touched it with my hand, I dared that. But if you were to step on it …’ He took a pebble and set it carefully on the sloping surface of the cylinder. It seemed to rest still, just for a moment, and then began to slide into the mouth of the cylinder, picking up speed gradually until it disappeared into the shadows. ‘See?’ Titus grinned. ‘You would fall on your backside and you would slither off out of sight, for ever.’

‘Not for ever,’ the ColU said. ‘Titus, I dare say you’ve tried this experiment a few times. When exactly did you drop your first rock down this shaft?’

‘Actually it was a spare torch, I wanted to see how far it extended …’

They compared times. Titus always kept a careful check on times when marching or scouting. He had dropped the torch about an hour and fifteen minutes earlier.

‘Good,’ said the ColU. ‘We won’t have long to wait.’

Stef frowned. ‘Wait for what? This enigmatic manner of yours is irritating, ColU.’

‘I’m sorry. When I was a mere farm machine, you know, people rarely listened to my speculations—’

‘Spill it, tin man.’

‘Colonel Kalinski, I think this is a gravity tunnel. It’s an old idea, dating back to contemporaries of Newton.’

‘Never mind the history lesson. Just tell us.’

‘Imagine a tunnel dug through the ground, in a dead straight line between two points on a planet’s curving surface. The tunnel is straight, but you can see that it will seem to dive down into the ground at one point, and then climb up again at the destination.’

Stef nodded. ‘I get it. So if you line the tunnel with a frictionless surface, and climbed on a sled—’

‘You would slide down into the ground, reaching some maximum speed at the midpoint of the tunnel, until slowing to the other end. It would feel as if you had descended a slope and climbed another, but in fact you would have followed the tunnel’s straight line all the way. Do you see, Stef Kalinski? The passage is energy free, once the tunnel is cut. Powered by gravity alone. And if you built a network of tunnels, and made them durable enough—’

‘You’ve built a transport system that could last a billion years.’ Stef grinned at the audacity of it. ‘All but indestructible, and free. I love it. So the people who built this, whether they were our descendants or not, must have been pretty smart.’

The ColU said, ‘They may not have been people at all. This is Per Ardua. Remember we had evidence that there was a builder culture that achieved planetary engineering. Maybe this is somehow a legacy of that.’

Titus was frowning. ‘I am trying to work this out. So my torch will have slid along this tunnel to the terminus. And then, with nobody to collect it – or so I presume – it will have started to slide straight back again. You say we must wait only a few minutes, ColU. Do you mean until my torch returns? But how can you know that? You don’t know how long this tunnel is …’

‘It doesn’t matter,’ the ColU said. ‘It’s an odd quirk of physics. The time the journey takes only depends on the density of the planet, the gravitational constant … Even if you could cut a tunnel right through the centre of the planet—’

‘Which would have been handy getting from substellar to antistellar,’ Stef said drily.

‘Even then, though you’d have reached much higher speeds at the midpoint, the journey time there and back would be the same.’

Titus said, ‘All this sounds like philosophical trickery to me. And how long is this magic transport time you predict, o glass demon?’

The ColU said, ‘Just wait … About this long.’

And, right on cue, a bundle of reeds came sliding up out of the mouth of the tunnel. As it slowed to a halt, Titus carefully reached down and swept it up with his one good hand. ‘Ha! A fine trick, demon. But now we have some planning to do. Come! Let us return to camp.’

The first trip through the gravity tunnel, Titus decreed, was to be made by sled, Beth’s cart, with the runners they had made to replace the axles and wheels on the undersurface. Of course they had anticipated having to drag the cart over farside ice, but Stef could see that this arrangement ought to work even better in the frictionless tunnel.

So they wheeled the cart the couple of miles to the tunnel mouth, established a temporary camp, spent a day fixing up the cart with its sled rails. They ate and slept, according to Titus’s stern orders.

Titus decreed that the first to take a trial trip through the tunnel would be himself with his daughter Clodia – and the ColU and Stef, who might be able to interpret the experience, and what they found on the far side. The pregnant Mardina, the baby’s father Chu, and prospective grandmother Beth, would not be split up come what may; they would be staying behind.

They were evidently going to have to do some fancy work getting the crew loaded on at one end of the tunnel, and successfully off at the other before the sled started to fall back, without any outside help. Before they hauled the cart over to the tunnel Titus had them practise the art. They had most success with Titus and Clodia leaping out at the destination, carrying rope to tie up the cart, while Stef stayed in the cart cradling the ColU.

Then the cart crew bundled up in their warmest gear – they were after all going an unknown distance deeper into the chill of farside – and loaded food, water, blankets, material for a fire and a few of their precious tools onto the cart itself. Beth, Mardina and Chu had an easy enough time pushing the cart over the lip of the sloping tunnel, and held it steady while the passengers climbed aboard.

Then Titus ceremoniously lit a torch and held it aloft. ‘Onwards, and into the unknown!’

The support crew let go of the cart. Slowly, almost imperceptibly, it began to slip down the slope.