Текст книги "The Long Mars"

Автор книги: Terence David John Pratchett

Соавторы: Stephen Baxter
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Текущая страница: 20 (всего у книги 25 страниц)

37

TO SALLY, PILOTING Frank in Woden, it was just another dead Mars. As seen from a high altitude the basic shape of the landscape, the tangle of Mangala Vallis below, the great rise of the Tharsis uplands to the north-east, looked much as she remembered it from spacecraft images of the Datum-Earth Mars, taken decades ago in a reality all of three million steps away.

Behind her, Frank, sleepy, grumpy since they had run out of caffeinated coffee a week back, was also unimpressed. ‘What the hell can he have found, if even a new set of Commandments from God on those damn monoliths wasn’t good enough?’

‘It’s not visible to the naked eye,’ said Willis from Thor, his voice crackling over the comms. ‘I’ve had optical and other scanners searching for it, from both gliders.’

Sally said, ‘Tell us where to look, Dad.’

‘More or less east. You won’t see it, not from here. Use your screens . . .’

Sally fooled with her screen, looking in the direction he’d told her, exploring the bulging Tharsis province landscape under the usual featureless toffee-coloured sky. She saw a lot of horizontals, the uneven horizon itself, craters reduced to shallow ellipses by perspectives, gullies on the uplifted flanks of the volcanoes, all painted a monotonous brown by the ever-present dust. No odd shapes, no unusual colours. Then she allowed the software to scan the image for anomalies.

‘Oh, my,’ said Frank. Evidently he had done the same thing, about the same time. ‘I was looking at the ground, the landscape. The horizontals.’

‘Yeah. When all the time . . .’

There was a vertical line, a scratch of very un-Martian powder blue, so fine and straight and true it looked like an artefact of the imaging system, a glitch. It rose up out of the landscape from some hidden root. Sally let the image pan, following the line upwards. What was this, some kind of mast, an antenna? But it rose on up into the sky – up until the imaging system reached the limit of its resolution, and the line broke up into a scatter of pixels, still dead straight, fading out like an unfinished Morse code message.

Frank said reverently, ‘Arthur C. Clarke, you should be seeing this. And, Willis Linsay – respect to you, sir. You found what you were looking for, all this time. I get it now.’

Willis said, only a little impatiently, ‘OK, let’s get the fan-boy stuff out of the way. I take it you understand what you’re looking at.’

‘A beanstalk,’ Frank said immediately. ‘Jacob’s ladder. The world tree. A stairway to heaven—’

‘What about you, Sally?’

Sally closed her eyes, trying to remember. ‘A space elevator. Straight out of those wonders-of-the-future books you used to give me as a kid.’

‘Yeah. Future wonders of my own childhood, actually. Well, here it is. A cheap way of getting to orbit, basically. You put a satellite in orbit to be the upper terminus of your elevator string. You need it to hover permanently over the lower terminus, which is on the ground. So you put it over the equator, or close to, at an orbit high enough that its period matches the rotation of the planet.’

‘Where they station the communications satellites.’

‘Right. Mars has about the same day as Earth, so a twenty-four-hour orbit does the trick here too. Then you just drop a cable down through the atmosphere—’

‘The engineering details of that,’ Frank said dryly, ‘are left as an exercise for the reader.’

‘Then you fix it to the ground station, and you’re in business,’ Willis said. ‘Once it’s in place, no more expensive, messy rockets to get off the planet. You get a cable-car ride to the sky, fast, cheap, clean. In principle this technology will work on any world. Any Mars. This Mars is better than our own, in fact, because it doesn’t have any pesky low-orbit moons to get in the way.’

Sally was plodding through the logic of this situation. ‘Let me get this straight, Dad. You predicted you were going to find a space elevator on Mars – I mean, somewhere in the Long Mars. How did you know? Who built it? How old is it? And why do you want it?’

‘How did I know? It was a logical necessity, Sally. Any advanced society on a Joker Mars is going to strive to reach space, before the window of habitability closes, as close it must. And if a spacegoing culture does arise, then a space elevator is going to be something they’re going to reach for, because it’s so much easier to build here on Mars, than on Earth. Who built it? Irrelevant. Somebody was bound to, given enough time – enough chances, in the worlds of this Long Mars.

‘As to why I want it – look, we need this back on Earth.

‘The big challenge for a space elevator is getting hold of a cable material strong enough. On Earth, you’d need a cable twenty-two thousand miles long, and said cable has to hold up its own weight, against the pull of gravity. If you used fine-grade drawn steel wire, say, you’d only be able to raise your cable through thirty miles or so before it would pull itself apart like taffy. That’s a long way short of twenty thousand miles. In the old days there was much fancy talk of special materials with a much higher tensile strength – graphite whiskers and monomolecular filaments and nanotubes.’

‘You understand this was all before Step Day,’ Frank said. ‘When because of you, Willis, everybody got distracted by travelling stepwise instead of up and out, and the dreams of opening up space were abandoned.’

‘OK, my bad. But, Sally, the point is that building an elevator on Mars is much easier than on Earth. The lower gravity, a third of Earth’s, is the key. Satellites orbit a lot slower than around Earth, at a given altitude. So the twenty-four-hour synchronous orbit is only eleven thousand miles up, not twenty-two. And you can use materials of much less tensile strength to make your cable. You see? That’s why space elevators are a much more accessible technology on Mars than on Earth. But if we can take this cable stuff home – learn its lessons, retro-engineer it to find out how it works, enhance its performance for Earth’s conditions – we’ll skip decades of development and investment.

‘Think about it. What a gift for humanity, just when we need it. Once you have an elevator, access to space is so easy and cheap that everything takes off. Exploration. Huge developments like orbital power plants. Resource extraction, asteroid mining, on a vast scale. Some of the Low Earths have populations of tens of millions now, since the Yellowstone evacuations. And as they industrialize, if they start with easy access to space, they’ll be able to keep it clean and safe and green from the beginning. We could have a million-fold industrial revolution across the Long Earth, on worlds as clean as my garden in Wyoming West 1, Sally, where you used to walk me as a kid. And as for the Datum itself, given the depletion of oil and coal and mineral ores there, this is the only way the old world can ever recover.’

‘You are playing Daedalus again, aren’t you?’ Frank said. ‘I guess the historians will call it Beanstalk Day this time.’

‘Things have a way of working out. Stepping did, didn’t it?’

‘Sure. After a slew of social disruption, economic chaos—’

‘And a billion lives saved during Yellowstone. Whatever. Anyhow this conversation is irrelevant because—’

Sally said, ‘Because you’re going to do this anyhow.’

‘Yep. Come on, let’s head over; I want to find the root station before it’s dark. Then we’ll need to figure out how to acquire some kind of samples to take back. The cable is the thing; if we get pieces of that material the rest is detail.’

Sally pushed at her joystick; the glider climbed higher, banking to the east. ‘One more question, Dad. So you figured that somebody would have come up with the space elevator idea, somewhere on the Long Mars. All you had to do was keep stepping until you found it. But how did you know it would be here? I mean, geographically. If I understand it right you could grow a beanstalk anywhere along the Martian equator.’

Frank said, ‘Let me try to answer that one. We’ve been tracking the big Tharsis volcanoes. Right, Willis? Stick a beanstalk on top of Olympus Mons and you’re already thirteen miles up towards your goal, and above eighty per cent of the atmosphere, thus avoiding such hazards as dust storms.’

‘Actually Pavonis Mons would be a better choice,’ Willis said. ‘Not as big but slap on the equator. Yes, Frank, that was how I figured it; Tharsis had to be a site, if not the only one . . . Hmm.’

‘What?’

‘I’m getting better visuals now. Up here, out of the dusty air. As it happens the cable line doesn’t quite line up with the summit of Pavonis. Engineering details. Soon we’ll know for sure. Come on.’

They flew on, Sally tracking Willis, heading steadily east, away from the setting sun, over slowly uplifting land. The shadows speared out from the rocks and pooled deep in the craters, where Sally imagined she saw mist gather.

At last she thought she could see the cable itself with her naked eye, a baby blue scrape down a sky turning a bruised purple. She tilted her head, watching it spear up, up out of her vision, impossibly tall.

‘Like a crack in the sky,’ Frank said. ‘What’s that old song?’

‘It makes me feel kind of giddy,’ Sally said. ‘In an inverted way. I’m glad I can’t see the anchor satellite, poised up there. What if this thing broke and fell?’

‘Well, the cable would wrap around the planet as it rotated, and cause a hell of a lot of damage. There was a novel called Red Mars—’

‘It’s not going to fall,’ Willis said.

‘How do you know?’ Sally snapped.

‘Because it’s very ancient. If it was going to break and fall, it would have done so by now. Ancient, and lacking maintenance for a long time.’

‘And how do you know that?’

‘Look at the ground below.’

The featureless plain was scattered with meaningless shadows. No structure, Sally realized. No sign even of a relic.

Willis said, ‘Think where we are. At the foot of a space elevator, this should be the hinterland of a port that serves a major chunk of the planet. Where are the warehouses, the rail lines, the airports? Where’s the city to house the travellers and the workers? Where’s the farmland to feed them all? Oh, I know whatever race built this probably had totally different ways from the human of solving those problems. But you don’t build a space elevator unless you want to bring materials down from space, or ship goods back up into space, and you don’t do that without some kind of facility to handle stuff on the ground.’

‘And there’s nothing down there,’ Sally said. ‘How much time, Dad? How much time to erode everything to invisibility?’

‘I can only guess. Millions of years? But the elevator survived all that time, the dust storms and the meteor impacts – and its own exotic hazards, such as solar storms and cable-snipping meteors further up. Whoever built that built it well . . .’

Suddenly the wonder of it hit her, the strangeness of the situation. Here was the product of a long-vanished indigenous civilization, about which Willis could have known nothing. Nothing about their nature, the detail of their lives – their rise, their fall, their evident extinction. And yet, from the sheer planetary geometry of Mars, he had deduced they must exist, or must have existed, and they must have built a space elevator. And he was right, here was that final monument, their last legacy, with everything else about them worn to dust. As if they had only ever existed for this one purpose, to fulfil Willis’s ambition. And he, in turn, had crossed two million Earths, the Gap, and three million copies of Mars, in the utter certainty of what he would eventually find. Not for the first time in her life she wondered what it must be like to live inside her father’s head.

‘OK,’ Willis said, ‘we’re coming up on the base of the cable. We’re still a ways short of Pavonis Mons. I guess the base could have been relocated . . .’

The gliders dipped towards the ground. They lit up the darkling landscape ahead with their searchlight beams, and Willis fired off a couple of flares. The artificial light made the cable gleam, a mathematical abstraction above the chaotic jumble of the plain.

At last Sally saw where the cable touched the ground – but it did not stop there. The blue line dived down into a circle of darkness, foreshortened from this distance. At first Sally thought it was a crater. Then, as the gliders flew overhead and looped past the cable itself, she realized she was looking down into a hole, a shaft that might have been a half-mile wide – smooth, symmetrical, a well of darkness.

Willis growled, ‘I pinged it with my radar. That’s where the cable goes, all right; that’s where the root station is. Down there. Damn thing is over twenty miles deep.’

That shocked Sally. ‘How deep?’

‘Deep enough to contain a decent thickness of air.’

Frank the trained astronaut took over. ‘Deep enough that we wait until the morning before taking a look inside.’

Willis hesitated. Sally knew his instinct would be to uncoil a rope and just plunge down there with a flashlight, Martian night or no Martian night. But at length he said, ‘Agreed.’

Frank said, ‘You hotshot pilots just make sure you don’t run into that cable on the way in to landing. I’m guessing that if this thing has lasted as long as you say, Willis, then if we pick a fight with it our gliders are going to come off worst . . .’

And as they came down, Sally thought she saw a light in the landscape, off in the distance, far away from this beanstalk root. A single light in the dark that was extinguished when she looked again. If it had ever existed at all.

38

IN THE MORNING the three of them resolved to hike to the pit, leaving the gliders behind. That was basically Frank and Willis’s plan. A plan that entailed leaving the gliders unguarded . . .

Sally didn’t contribute much to the discussion. She was doubtful about the plan, however. This was Mars, a typical Mars – a dead Mars, aside from whatever they were likely to encounter in the pit. There were no real hazards here. Even a dust storm, pushed by Mars’s feeble air, would barely leave a mark of its passing. The only real danger was an unlucky meteor strike, and no sentry could ward off that. To post a guard, thus splitting their tiny team, would have been absurd.

Wouldn’t it?

Sally was cautious by nature; living alone in the wild worlds of the Long Earth had made her so, long ago. But her caution was of a different degree to Frank’s. He thought in terms of physical effects, equipment failures – a meteor strike, a solar flare, a leaky pressure hull. While Sally had learned to think in terms of malevolent life – creatures out to kill her, one way or another. Maybe she was importing an over-caution bred on a too-alive Earth to a too-dead Mars where it wasn’t appropriate. Maybe this was just a distraction.

Wasn’t it?

She went along with the guys’ plan. But in her head a small alarm sounded softly, continually.

And she remembered that light she’d thought she’d seen, glowing in the Martian night.

So the three of them walked to the pit. In the bright daylight the thread of the cable was even more striking than in the twilight, a brilliant eggshell blue like no natural colour Sally had seen on any of the millions of Marses they had visited.

As they walked, Willis held up a small sensor pod to study their target. ‘That cable is about a half-inch thick,’ he said. ‘A finger’s width. You know, I’m betting it doesn’t need to be that thick.’

‘A safety factor,’ Frank suggested. ‘Maybe the apparent thickness is mostly dummy, a lightweight safety coating. You don’t want to be slicing off the wing of your flying machine—’

‘Or your limbs—’

‘On a super-strong thread that’s too fine to even see.’

As they talked Sally was studying the ground, the lip of the approaching pit. ‘No raying.’

‘What?’ Frank asked.

‘No splash debris, like from any other crater on Mars, or the moon.’

‘Umm,’ Frank said. ‘But there is a crater wall, of sorts . . .’

The ground rose up as they neared the lip, hard-packed under the dust, to become a circular barrier maybe fifty feet tall, Sally saw as she crested it, a wall that ran right around the rim of the hole in the ground. This was a big feature, it was obvious now they were standing on top of it, a hole a full half-mile across encircled by this smooth wall. Away from this highest point, which was a broad ridge so Sally had no fear of falling, the lip fell away smoothly, funnelling into the ground. From here she could only see the upper sections of the interior walls of the pit itself, which looked like compacted Martian rock.

Willis cautiously knelt down, tied a fine rope to a handheld sensor pod, and lowered it into the pit, paying out the rope, clumsy with his gloved hands. ‘Yeah, this pit is indeed just about twenty miles deep; the radar confirms it. And pretty much the same radius all the way to the bottom. It’s a cylinder.’

Frank said, ‘Surely no meteor could create a pit as deep and orderly as this. A bigger impactor doesn’t drill a deeper hole, it just melts more rock, and you get a wider, shallower crater.’

‘Hmm,’ Willis said. ‘I can imagine how it could be done. A string of small impactors coming down one after the other. Deepening the hole before it had a chance to infill.’

Frank pulled a face, looking dubious. ‘Maybe. If this is artificial I can think of easier ways to build it. Like with a massive heat weapon. Like we saw used in war, back on world – what was it?’

‘About a million,’ Willis said. ‘The Martian Arecibo.’

‘But,’ Sally said, ‘that’s a long way from here, stepwise. We’ve seen no evidence of cross-stepwise transfer of technologies, or even life forms, here on Mars.’

‘True. But convergence of technology types isn’t impossible,’ Willis said. ‘We have directed-energy weapons, and we’re not even from Mars.’

Sally shook her head. ‘We’ve got nothing but guesses. Why would anybody build this, though?’

Willis was monitoring the results coming back from his sensor pod. ‘I can make a guess at that. This pit is deep. The Martian atmosphere’s scale height is only around five miles. At twenty miles deep, you’d expect the air pressure to be around fifty times its value on the surface. Up here you have a typical Martian-surface atmosphere, a scrape of carbon dioxide at about one per cent Earth’s sea-level pressure. At the bottom of this pit, and my instruments are confirming it, that’s up to about fifty per cent.’

Frank whistled. ‘That’s better than on the Gap Mars.’

‘Right. Which is about as hospitable as we’ve found it, anywhere across three million stepwise copies. That’s why they built this pit, Sally. As a refuge.’

‘From what?’

Willis said, ‘From the collapse of the air. Maybe there was something like a volcano summer here – a deep one, a long one—’

Frank said, ‘Long enough for some breed of Martians to come up with a space programme.’

‘Right. But, like all summers, eventually it came to an end. The heat leaked out, the snow started falling at the poles, the oceans froze over and receded. The usual story.’

Sally thought she saw it now. ‘This pit is a refuge.’

‘Yeah. And it couldn’t be simpler. The pit would keep its air, water, even if civilization fell.’

Frank said, ‘And the elevator?’

‘Maybe they moved the root station here, before the end, from Pavonis or wherever else. Kind of romantic, but very long-term thinking. They lived in a hole in the ground to make sure they saved their air and water, but they kept their ladder to the planets.’

Sally peered into the pit. ‘So what’s down there now?’

‘Life,’ said Willis. ‘I can tell that much. There’s oxygen, methane – the atmosphere is unstable, chemically. So something must be photosynthesizing away, pumping all that oxygen into the air.’ He glanced around, at the way the slanting morning sunlight caught only the upper surface of the pit walls. ‘No, not photosynthesis. Not primarily anyhow – not enough direct light, in the depths. Maybe it’s like the deep-sea organisms on Earth, out of sight of sunlight, feeding on seeps of minerals and energy from underground. We’re close enough to the Tharsis volcanoes for that to work; the big magma pockets under those babies must leak a lot of heat.’

Sally asked, ‘So this is the last refuge of their civilization. Where’s the city lights, car exhausts, radio chatter?’

‘None of that, I’m afraid. There is one splash of metal.’

Frank looked startled. ‘Metal?’

‘An irregular form. Down on the floor of the pit.’

Sally said wistfully, ‘All this makes me think of Rectangles.’

Willis wasn’t interested, but Frank glanced at her. ‘Where?’

‘A Long Earth world I discovered with Lobsang and Joshua. We called it Rectangles, for the traces of foundation ruins we found on the ground. Another site with relics of a vanished civilization.’

‘Right. And a cache of high-tech weapons.’

She looked at Frank in surprise. ‘How did you know that? Oh. Jansson told you.’

‘We spoke a lot. Especially when she was in her last days, during Yellowstone. Told me a lot about her life. Her time with you—’

‘We’ll have to go down,’ Willis said, cutting across their talk. ‘Into the hole.’

Sally took a breath. ‘I was afraid you’d say that.’

‘In the spirit of noble exploration, I suppose,’ Frank said.

‘No. So that I can get up close and personal with that cable. And get a look at the root station.’

‘OK,’ Sally said dubiously. ‘Suppose, hypothetically, we agree we’re going to do this. How? We don’t have twenty miles of rope – do we, Frank?’

‘No. Anyhow we’d need a lot more, for doubling up, fail-safes.’

‘We don’t have winches, or jet packs—’

‘We fly down,’ Willis said. ‘We take one of the gliders, and fly down.’ He looked at them both. ‘You’re going to say no, aren’t you? Look. You can see how wide this pit is. A half-mile across – plenty of room for a spiral flight, down and back up.’

‘The air at the base is a hell of a lot thicker than the design optimum, Willis,’ Frank protested.

‘You know as well as I do that fifty per cent bar is still within the performance envelope. And besides, there’s a lot of heat seeping out of this hole in the ground. We can ride back up using the thermals; that will help.

‘Here’s the plan. Two of us will ride one glider down, leaving the other glider on the surface as backup, together with one pilot. We can offload stores before the flight. There are obvious fallback strategies, if anything goes wrong. Maybe we could even climb back up, out of this pit. The gravity is a baby.’

Frank said, ‘Why not send down a drone plane?’

‘Not equipped to take samples.’

‘But—’

‘End of discussion,’ Willis said. ‘We came here for that damn space elevator. We ain’t going home without a piece of it. Got that? OK. Let’s get down to specifics.’

They argued about how to split the crew. They agreed that one should stay on the surface, two descend. Which one, which two?

In fact the logic was clear. Willis was always going to go into the pit. Sally was the least good pilot, but as the youngest and fittest she had the best chance of climbing out of that hole in the ground if things got bad enough. Frank, meanwhile, the best pilot, was the obvious choice for the reserve on the surface.

Willis and Sally it would be, then.

Willis fretted through the day that Frank insisted they took in offloading Thor, the glider to be used for the descent, testing through its systems one more time, checking over their pressure suits and other gear, working out communications protocols and the like. And if Willis was restless, Frank was visibly unhappy, whether because the stunt was so obviously dangerous or because he was the guy left behind to mind the store, Sally wasn’t sure.

Come the evening they had a hot meal in one of the bubble tents, washed up, and took to their sleeping bags early. The plan was to rise at dawn and use the full day to descend, do whatever had to be done at the base of the pit, and climb back out again before the sun fell.

That night Sally slept no better and no worse than she had during the whole trip. Another legacy of her solitary, nomadic life: she had adapted to getting by on whatever sleep she could snatch, as and when she got the chance. She was always aware, though, oddly, of the thread to the sky just a couple of miles away, silent, ancient, with space at its tip and some kind of fallen culture at its feet. Her life had always been odd, even before Step Day. Just when she’d thought it couldn’t get any odder . . .

Thor lifted, propelled by the methane rockets, as obedient and responsive as ever. Willis was piloting.

Once they were into their glide Willis made one circle over the landing site. Sally looked down at the ground, at Woden gleaming bone white in the morning sun, and their bubble tents like blisters on the scuffed Martian dust. Frank Wood stood alone, staring up. He waved, and Willis waggled the wings in response.

Sally still had that faint alarm bell ringing in the back of her head. There was something about this situation that wasn’t right, that they hadn’t thought through or prepared for. Well, Frank Wood was more experienced than Sally in this kind of situation, less intelligent than Willis maybe but calmer, more capable in many ways. If something did blow up, she’d have to rely on Frank’s instincts to save the day.

Thor turned away from the landing site and towards the pit, and Sally turned her attention to the challenge facing her.

They were over the pit in only a couple of minutes. Willis, getting the feel of the craft, took Thor banking in tight circles over the opening, keeping one eye on the elevator cable. ‘I can see the cable easily,’ he said with some relief. ‘Also I rigged up a proximity sensor that will ping if we get too close. Short of flying straight at the damn thread, we should be OK.’

‘Don’t tempt fate, Dad.’

‘Now you sound like your maternal grandfather, Patrick. Remember him? The gloomy Irishman. OK, let’s take her down.’

He began a lazy spiral around the axial cable, cutting the speed, Sally guessed, as low as he dared without risking a stall. Soon they were descending towards the mouth of the pit, the low sunlight wheeling through the glider cabin – and then, with a smooth wash of rising shadow, they fell beneath the lip of the hole, with its artfully consolidated ridge. The sun caught only the uppermost stretch of the wall of crimson rock, and soon they were falling into the darkness.

Sally felt an odd sense of claustrophobia. But that was logical, for her, with the instincts of a natural stepper. Sally had grown up knowing in her bones that as a last resort, whatever difficulty she got into, she could always just step away, even without a Stepper box. Even on the Long Mars that was true, though she would generally just be swapping one lethal landscape for another. But you couldn’t step out of a pit, a hole dug into the ground, because there would be earth and bedrock in the worlds to either side stepwise. A pit, a basement, a cellar, even a mine, was therefore a simple defence against stepping aggressors, as had been figured out very early after Step Day, even by neighbourhood cops like Monica Jansson.

On the Long Mars as on the Long Earth.

She was trapped in a cage one world thin.

As she descended twenty miles.

Into the dark.

Towards the unknown.

It came as a relief when Willis switched on lights, shining front and back of the glider and to either side, picking out the wall on the one hand and the cable on the other. The floor was still too far below to be visible. The wall of the pit was layered, with a spray of sun-blasted dust on the surface, then a mass of rubble and gravel and ice – and then the bedrock, itself deeply cracked, a record of the huge primordial impacts that had shaped this world. She wondered if these walls had needed some kind of consolidation, to keep this tremendous shaft from collapsing. Maybe Mars’s lower gravity, and its cooler interior, helped with that.

‘Piece of cake,’ Willis said as he piloted the glider. ‘Just got to hold her steady. And get used to the thickening air. Worst danger is I’ll fall asleep at the wheel.’

‘Don’t even joke about it, Dad.’

‘You keep watching, the walls, the ground. I have cameras working and other sensors, but anything else you spot—’

‘I can see something.’ The wall, in the plane’s spotlight, was no longer featureless, she saw. The rock face, as rough as ever, was etched with a kind of zigzag spiral. ‘Stairs,’ she said. ‘I see stairs. Big ones, four or six feet deep, it’s hard to tell from this vantage. But they’re stairs, all right.’

‘Ha! And we’re not a mile deep yet. Should have anticipated stairs. A culture careful enough to build this hole in the ground in anticipation of its entire civilization collapsing was always going to install something as simple as stairs.’

‘Why don’t they reach all the way to the surface?’

‘Maybe they just eroded away. I have the feeling this pit has been here a long time, Sally.’

After that, for a time they descended in silence. The circle of Martian sky above them receded, a coppery disc, like a coin. From above, the ship must look like a firefly spiralling down the barrel of a cannon. Still the base of the pit was invisible.

At about twelve miles deep Sally thought she saw more detail on the wall, and she had her father level out for a closer look.

‘Vegetation,’ she said, watching carefully as the glider slid past the walls. ‘Stumpy trees. Things like cacti. Dad, this is like what we saw on Gap Mars.’

He checked the air pressure. ‘Yeah, we’re up to about ten per cent of a bar already. I guess this is the lower limit of tolerance for that vegetation suite. And there must be just enough sunlight down here to support their kind of photosynthesis. Remarkable, isn’t it, Sally? We keep seeing the same biospheric suite, essentially, taking its chance wherever it can, wherever the environment lets up its stranglehold, even just a little. I can feel the air thickening, getting kind of bumpy. . .’

So it was. Sally guessed that the pool of air trapped in the pit was turbulent, stirred up by the heat from below and falling back when it cooled. She tried to watch for more evidence of life on the walls, but mostly she monitored the glider’s increasingly ragged descent.