‘It’s quite a sight, Joshua,’ Lobsang said now. ‘The caldera. Even for hardened High-Meggers travellers like us. And here it is on the Datum. Horrifying if you think about it.’

He, or rather an ambulant unit, sat with Joshua, in orange robes and with shaven head, and with a rather immobile artificial face, Joshua thought. And his capacity for small talk hadn’t improved either. Still, here they were.

Joshua cradled a coffee infinitely stronger and more flavourful than the one he’d been served in Twin Falls, and looked down at the cleared highway below, a band of black cutting across a grey-white landscape. A few trucks moved between the surviving townships, but he also saw horse-drawn buggies, like something out of an open-air museum. Bicycles, too, at least close to the towns. Even what looked like a dog sleigh, cutting across the snow banks. ‘Quite a sight,’ he said. ‘Ten years ago you would never have believed you’d see all this.’

‘Indeed. It is as if the climate bands have suddenly shifted a thousand miles closer to the equator, from north and south. So that Los Angeles, say, now has a climate similar to Seattle’s before the eruption.’

‘I know. I’ve been there. The Angelinos just hate all that rain and fog.’

‘While Seattle itself is more like Alaska. Much of the planet north or south of forty degrees, in fact, has been largely abandoned to the ice. Canada, north Europe, Russia, Siberia – empty, the nations collapsed, the people gone stepwise, ancient cities deserted save for hardy hold-outs. Nelson Azikiwe tells me that little moves in Britain now save for salvage parties from the Low Earths trying to rescue cultural treasures.’

‘Nelson Azikiwe?’

‘Another of my friends, Joshua. Actually you met him in my reserve in Low Earth Madison on the day of the eruption. I’d like you to link up with him, in fact.’

Joshua didn’t respond to that. For ‘friends’ read ‘assets’. Sometimes he felt as much a ‘friend’ to Lobsang as a chessboard pawn would to a grandmaster. Even so, ultimately he’d probably find himself doing what Lobsang asked.

‘The politics of the Datum Earth has been dramatically reconfigured,’ Lobsang said now. ‘The new powerhouse nations are Southern Europe, North Africa, India, south-east Asia, southern China – even Mexico, and Brazil which is exploiting the final dieback of the rainforest to open up Amazonia to agriculture and mining. There is much jockeying for position in the new order, as you can imagine. China is somewhat disconnected from its stepwise footprints, compared to America and its Aegis, but on the Datum the Chinese are very strong.’

‘Good luck to them.’

‘But Datum America is prostrate. Not that this concerns you overmuch, I imagine, out in your homestead at Hell-Knows-Where.’

Joshua scowled. ‘You know damn well that I don’t live there any more, Lobsang. I haven’t even been back there in months. You had to send out Bill Chambers to fetch me back from my latest sabbatical, didn’t you?’

‘I had hoped that you might have been able to come to some reconciliation with Helen.’

‘Then you don’t know Helen. I guess all the time I spent back here at the Datum after Yellowstone was the final straw – even though she knew it was the right thing to do. I could never get the balance right, not for her, between home, and—’

‘And the call of the Long Earth. The two sides of your nature.’

‘Something like that.’

‘And Dan?’

‘Oh, I see him as much as I can. Fine kid – thirteen years old, and already taller than me.’

‘And yet your sabbaticals still draw you away . . . How’s your hand, by the way?’

Joshua lifted his prosthetic left hand to his throat, pretended to choke himself, and pretended to fight it off. ‘It has good days and bad days.’

‘I could get you something far better, you know.’

‘With you inside? No offence, Lobsang – but no.’ He held out his mug. ‘Have we run out of coffee already?’

The airship travelled at a leisurely pace. It took until evening before they were over Idaho Falls, maybe eighty miles from the caldera. Here Lobsang said they would stop for the night.

At Joshua’s request Lobsang lowered the ship so they could climb down to the ground, briefly escaping from the heated air of the gondola, although Lobsang insisted they should return to the airship before dark: ‘A lot of bandits out here nowadays, Joshua.’

With Lobsang at his side, Joshua walked around experimentally on a road surface choked with ice and ash drifts, and peppered with boulders of pumice so massive it was hard to believe that any force could propel them eighty yards through the air, let alone eighty miles. The air was bitterly cold, attacking his cheeks and nose and forehead, any part of him that was left exposed by the layers of his cold-weather gear.

He came to a stream, flowing sluggishly. The water was grey with ash, and the tree trunks by the banks were grey-brown. The scene was eerie, the light coppery as the sun went down. And the world was silent. There had been no traffic on the interstate for many miles, but nature was subdued here too; Joshua heard not so much as a bird call, as he inspected the spindly trunks of dead pine trees.

‘Kind of quiet,’ he said to Lobsang.

The ambulant unit was kitted out in Arctic gear as he was. Its breath, evidently heated and kept moist by some internal mechanism, steamed quite convincingly, a touch of verisimilitude. ‘The world is quieter still for me. So many communications nodes and networks have failed or been abandoned. To me, Joshua, it is as if the world is becoming Thulcandra.’

Joshua knew the reference. ‘The silent planet. Why did you bring me here, Lobsang?’

‘How’s your headache?’

‘Of course you’d know about that. If you want to know, it’s worse than ever. I mean, I usually feel uncomfortable when I’m at the Datum, or close to it, but this is worse . . .’ He tailed off, glancing around. He thought he had heard something, breaking the deadened silence. A furtive scuffling. A wolf, starving in this frozen wilderness? A bear? A human, some kind of bandit, as Lobsang had warned him?

Lobsang seemed unaware. ‘But this is different, yes? Your headache. You must have a sense that something about the Datum has changed.’

Joshua grunted. ‘And so do you, right? You’ve got evidence, haven’t you? Evidence of something. Otherwise you wouldn’t have called me back.’

‘Indeed. Evidence of something – well put. Something elusive and difficult to define, yet nevertheless apparent to me, who, despite my post-volcano handicap, still spans the world like a disembodied bardo spirit—’

‘Like a what?’

‘Never mind. Something real, Joshua. Look – you know me. If nothing else I am a keen student of the folly of mankind, which at times has seemed almost terminal.’

‘As we’ve discussed many times,’ Joshua said dryly.

‘Well, now something has changed. The aftermath of Yellowstone seems to have triggered it. People have responded well or badly. But amid the heroism and cowardice, the generosity and the venality, if you take a global view – and I am scarcely capable of less – it seems to me that humanity’s response to Yellowstone has been characterized by a startling outbreak of what Sister Agnes once described as common sense.’

And, just as he uttered those words, a figure in an orange jumpsuit, barefoot, with shaven head, materialized out of thin air, already in the middle of a flying leap. ‘HAAARRRGGH!’

‘Not now, Cho-je!—’

But Lobsang’s words were cut off as the newcomer wrapped his legs tight around Lobsang’s neck. Lobsang was forced to the frozen ground – but as he fell he stepped away, disappearing, leaving the newcomer rolling alone in the dirty ice, ash staining his orange jumpsuit.

Joshua carried a gun, bronze, steppable. He always carried a gun. Before the guy could stir Joshua had the weapon out in front of him, held two-handed, legs wide. ‘I knew I heard something tracking us. Don’t move, grasshopper.’