The rocking began to reduce.

“Take it easy,” the pilot said.

The Chinook levelled out.

Henry looked back again. The cloud was still expanding faster than they could run, probably three hundred miles an hour or better, close enough now to turn the day dark.

There was a patter of ash particles on the windscreen.

“North,” he said. “Go north.”

The pilots hurled the Chinook into a sickening wrench to the left.

Henry looked back. The cloud was still expanding, but mostly southward; the north was shielded a little by the topography of the Seat, what was left of it.

The cloud was separating into distinct mushroom-shaped clouds, thick and black, heavy and pregnant with ash. There were lighter cirrus clouds arrayed above. He could see the ashfall beginning already, a black ram; it would turn what was left of Edinburgh into a new Pompeii, he thought.

The Salisbury Crags, at the western face of the Seat, had given way. He could see what looked like a pyroclastic flow, a heavier-than-air mix of gases and hot volcanic fragments. From here it looked like a smoke ring spreading down the battered western flank of the Seat. The flow would follow the contours of the ground, and pool in the lower areas: the heart of Edinburgh, the old loch that had been drained to build the New Town.

Already the Seat itself, what was left of it, was scraped bare of life. And, through the clouds, he couldn’t see any sign of the Moonseed pools.

Lightning bolts shot through the clouds, extending to tens of thousands of feet.

For Morag, it started with a low rumble, like a tram deep underground. But there were no trains running today.

Then a series of buffeting jolts. Jolts that grew stronger.

She stood in the middle of Princes Street, in the roadway away from the buildings.

Then she was down, her face slammed against the tarmac. It was as if a rug had been pulled away from beneath her feet.

She tried to get to her knees. There was blood on the tarmac, a deep sting down the right side of her face, where the skin had been scraped away.

The noise was suddenly enormous, the crashing and roaring of the buildings overlaid on the deeper rumble of the ground. There was a muddled stink, of gas, steam, ozone, soot.

The street, still shaking, was turning into a battlefield. The facings of the buildings were coming away and crashing to the pavements, sheets of stone and bright plastic and metal and glass, smashing and splintering as they fell, as if the street was imploding. Billboards and neon light tubes turned themselves into deadly missiles, showering shrapnel over the pavements below. Some of the older buildings seemed to be collapsing already, the breaking of their beams like gunshot cracks.

To the east, towards Arthur’s Seat, she could see red flames rolling and leaping, a growing pillar of black smoke. Sometimes the flames seemed to pause, to weaken, but then they would find new vigour and hurl themselves even higher than before. There was a constant muffled roar. It was like watching some immense oil refinery burning up.

She sat down in the middle of the road, keeping her hands away from the glass fragments skidding there. The road surface was cracking open — to her left, a great section of it was tilting up — but she seemed to be in a stable place, here, and far enough from the cracking facades of the buildings to survive that, with a little luck.

Hell, she thought. She might actually live through this. If she found somewhere to report in she would have a tale to tell…

But now there was a new explosion. A sound like a thousand cannon.

A wall of flame — taller than any of the buildings, laced with black smoke and steam — poured into the eastern end of Princes Street. It was almost beautiful, like a moving sculpture of smoke and fire and light. At the cloud’s touch buildings exploded like firecrackers, a blizzard of stone and metal and glass. Oh, shit.

Time turned to glue; she seemed to be able to make out every detail.

The flame hit the big old buildings at the end of the street. She saw the Register House’s portico and clock towers burst outwards, before fire erupted from within, white and hot, overwhelming the Wellington statue, the old Iron Duke on his horse, in the instant before the cloud enveloped them.

Now a fountain of flame and smoke erupted from the entrances to the Waverley Market, the underground mall built around the Station, and the panels of its roof fluttered into the air like so many leaves. A brilliant light enveloped the Scott Monument, two hundred feet of carved Gothic foolishness, magnificent in the flames” underlighting for one last instant, before it too erupted into shards of stone.

A wind buffeted her, blistering hot.

The cloud poured along the street, more like a fluid than a gas. It must be full of ash, heavier than the air. Lightning cracked within it. It was carrying rocks, irregular, glowing boulders and sharp-edged fragments that probably came from buildings. Carrying all that stuff, she thought, was actually going to make it more efficient at scouring the city as it progressed.

This thing was going to scrape Edinburgh down to the bedrock, she thought, and then the Moonseed was going to eat that. But maybe it would be beautiful, in its way.

So much for reporting in.

She stood tall, facing the heat. She had time for an instant of regret before the cloud reared before her, and—

Soon, it seemed to Henry, the ash had reached a huge altitude, maybe fifty thousand feet. Ash, laden with Moonseed dust.

It obscured the view of Edinburgh, and maybe that was a mercy; he glimpsed the fiery hell of Arthur’s Seat, the husks of burning buildings all around.

The pilot said, “We couldn’t have outrun it, could we, sir?”

“I don’t think so. No.”

“We’ll have to go back to Leuchars. I don’t trust this bird after inhaling all that shit from the air.”

“That’s fine.”

“We’ll find another transport to get you to London.”

Henry looked back at the high ash. It would inject itself all the way up to the tropopause, ten miles above the Earth, where the decreasing temperature of the thinning air was inverted, making the tropopause into an invisible lid on the lower atmosphere. But such was the violence of the eruption, the ash from the Seat would surely break through the tropopause into the stratosphere beyond. There was no rain up there, nothing to wash the debris back down again.

The ash from the Seat would form a thin veil that would spread all the way around the planet, the heavier fragments slowly drifting back down to Earth. He thought he could see, as it reached towards the stratosphere, the steel glitter of Moonseed dust amid the ash.

The genie was out of its bottle now.

“Welcome to Mars, fellas,” he said.