Sally said, ‘I’m guessing the caldera’s collapsed.’

They all knew what that implied; after seven days everybody was an expert on supervolcanoes. When the eruption finally finished, the magma chamber would collapse inward, a chunk of Earth’s crust the size of Rhode Island falling down through half a mile – a shock that would make the whole planet ring like a bell.

‘Let’s get out of here,’ Joshua said. ‘I’ll lead you.’

It took only seconds for Joshua to step the Brewers out of the house, and safely over into the impossible sunshine of West 1.

And, just as Joshua stepped back into the Datum ash to help Sally with the mother, the sound from the caldera collapse arrived, following the ground waves. It was a sky-filling noise, as if all the artillery batteries in the world had opened up just beyond the horizon. A sound that would, eventually, wash around the whole planet. The old lady, propped up by Sally, her dressing gown stained grey, her head obscured by towels, whimpered and clapped her hands to her ears.

Joshua, in the middle of all this, wondered who the ‘sensible young lady’ in the pioneer gear had been.

The Bardo Thodol described the interval between death and rebirth in terms of bardos: intermediate states of consciousness. Some authorities identified three bardos, some six. Of these Lobsang found most intriguing the sidpa bardo, or the bardo of rebirth, which featured karmically impelled visions. Perhaps these were hallucinations, derived from the flaws of one’s own soul. Or perhaps they were authentic visions of a suffering Datum Earth, and its innocent companion worlds.

Such as an image of dreamlike vessels hanging in a Kansas sky . . .

The US Navy airship USS Benjamin Franklin met the Zheng He, a ship of the Navy of the new Chinese federal government, over the West 1 footprint of Wichita, Kansas. Chen Zhong, Captain of the Chinese ship, claimed to have concerns about the role he was expected to undertake in the ongoing relief effort in Datum America, and an exasperated Admiral Hiram Davidson, representing an overstretched chain of command – well, everybody was overstretched, as the fall of this disastrous year of 2040 turned into winter – had mandated Maggie Kauffman, Captain of the Franklin, to take time out of her own relief efforts to meet with the man and discuss his concerns.

‘As if I have the time to salve the ego of some old Communist apparatchik,’ Maggie grumbled in the solitude of her sea cabin.

‘But that’s what he is,’ said Shi-mi, curled up in her basket by Maggie’s desk. ‘You evidently checked him out. I could have done that for you—’

‘I don’t trust you as far as I can throw you,’ Maggie murmured to the cat, without malice.

‘Which is probably pretty far.’ Shi-mi stood and stretched with a small, quite convincing purr.

She was a quite convincing cat, actually. Save for the green LED sparks of her eyes. And the prissy human-type personality she embodied. And the fact that she could talk. Shi-mi had been an ambiguous gift to Maggie, from one of the equally ambiguous figures who seemed to be watching her career with an unwelcome interest.

Shi-mi said now, ‘Captain Chen is on his way . . .’

Maggie checked her status board. The cat was right; Chen was in the air. Chen had insisted that the two twains needn’t land to exchange personnel; he was crossing in a two-person light copter which could easily be set down inside the Franklin, he bragged, if the US ship opened up one of its big cargo bays. These new Chinese liked nothing more than to demonstrate their technical capabilities, especially over an America still prostrate two months after the eruption. Show-offs.

Distracted, Maggie glanced out of her cabin’s big picture window at this world, a Midwest sky big and blue and scattered with light clouds, the green carpet of a stepwise Kansas semi-infinite and flat beneath her – and all but unspoiled still, even on this Earth a single step away from the Datum. But more spoiled than it used to be. Before September, before Yellowstone, Wichita West 1 had been little more than a shadow of its Datum parent, scattered buildings of logs and blown concrete set out in a grid that roughly aped the Datum town plan. It had been typical of its type. Communities like this started out serving their Datum parents as sources of raw materials, sites for new industrial developments, and room for extra living space, sports and recreation, and so they necessarily followed their parents’ maps.

Now, though, a couple of months after the eruption, this version of Wichita was surrounded by a refugee camp: rows of hastily erected canvas tents full of bewildered survivors, the ground littered with heaped-up drops of food and medical supplies and clothes. Twains like the Franklin, stepwise-capable airships, both military and commercial, hung in the sky like blimps over wartime London. It was a grim third-world scene, in the heart of a stepwise America.

Of course it could have been a lot worse. Thanks to the almost universal ability that people had to step away into a parallel world from anywhere on the Datum, the immediate casualties of the Yellowstone eruption had been comparatively light. The refugees below had in fact been transferred from Datum camps they’d reached by conventional means, fleeing along Datum roads away from the central disaster zone, before being stepped away to cleaner parallel worlds. Datum Kansas was a relatively safe distance from the eruption site itself, which was over in Wyoming. But even this far out the ash was taking its toll, on eyes, on lungs. It induced conditions with names like ‘Marie’s disease’, a kind of ghastly slow suffocation – horrors that were becoming too familiar to everybody, and the medical tents on the ground were surrounded by lines of exhausted people.

Lost in reflection, with worries about her own responsibilities nagging at her – as well as her own ever-present doubts about how well she could fulfil those responsibilities – Maggie was startled by a soft knock on her door. Chen, no doubt. She snapped at the cat, ‘Standing orders.’ Which meant: Shut up.

The cat calmly curled up and mimicked sleep.

Captain Chen turned out to be a short, bustling man, pompous and self-important, Maggie thought on first impression, but evidently a survivor. He’d been a party official who’d kept his position through the fall of the Communist regime, and in the Zheng He had in fact gone on to command a prestigious voyage of exploration into the Long Earth. She referred to this as she made him welcome.

‘A voyage which you yourself, Captain Kauffman, might have emulated by now, if not for the unfortunate circumstance of the eruption,’ he said as he sat down, and accepted an offer of coffee from Midshipman Santorini, who’d shown him in.

‘You know about the Armstrong II? Well, I’m not the only one whose personal plans have been disrupted by this.’

‘Quite so. And we are the fortunate ones, are we not?’

After some preliminary chatter – he said his pilot for the crossing, a Lieutenant Wu Yue-Sai, was being looked after in the Franklin’s galley – he got down to business. Which turned out, it seemed to Maggie, to be irritatingly ideological.

‘Let me get this straight,’ she said. ‘You’re refusing to carry ballot slips for our presidential election?’

He spread his chubby hands and smiled. He was a man who enjoyed bringing complications into the lives of others, she thought.

‘What can I say? I represent the Chinese government. Who am I to intervene in US politics, even in a constructive way? What if, for example, I were to make some error – to fail to deliver the papers to one district or another, or lose a sealed ballot box? Imagine the scandal. Besides, from an outsider’s point of view, to hold an election in such circumstances seems frivolous.’