Blue stepped forward, onto a wider neck of ground. He tested every step, as if he was walking onto an ice floe. Ted followed a few yards behind, trying to stick to the footsteps Blue had left in the ash. Fixed to his hood Blue had a chest-mounted still camera and a small video camera — the kind they put in cricket stumps, Ted thought irrelevantly. Blue was working the still camera now, and talking patiently into a microphone inside his hood.

After maybe fifty paces Blue stopped.

Ted came to stand beside him. The neck of blackened turf went on some yards further, but Ted could see how cracked and fragile it was becoming.

“Notice how it’s not advancing,” Blue said.

“What?”

“The Moonseed.”

“Why?”

“Who the hell knows? Come on. Open your bottles and let’s make like we know what we’re doing.”

Blue crouched and, leaning as if reaching out of a boat, began poking at the Moonseed debris with stuff from his equipment pack. He had probes of metal that he scanned over the pool surface or pushed into it, taking data through wires into his backpack, muttering to his tape the whole time.

Ted squatted down beside him, his knees and calf muscles protesting. He got hold of Blue’s belt at the back, near where he had tucked his heavy geologist’s hammer. It was like holding a child, leaning over a rail. Blue didn’t protest.

Ted looked around, back the way they had come. He was on a neck of land like a spit protruding into a silvery sea. At the “shore” he could see the rubble, the ruined Moonscape suburb through which he’d had to clamber. It seemed a long way away. The closest intact building was a way away to the west, halfway up Castle Hill, a squat pile of sandstone that looked like it might once have been St Giles” Cathedral; the old church poked out of the landscape like a beached wreck.

He could see no other humans, in any direction.

In his gloved hand Blue had cupped a small sample he’d taken from the Moonseed surface. “Look at this now. Be careful. It is very delicate.”

Ted bent. He had to wipe the ash from his faceplate to see.

It was like a spider-web; or an autumn leaf; or the skin stretched over the bones of a child’s hand. A fragment of structure, with the finest of membranes stretched between hair-thin spars.

He grunted. “Like something the Wright Brothers might have dreamed of.”

Blue laughed. His hand shook, just slightly, but it was enough to shake the fragment to pieces, to silvery Moonseed dust, which fell through his fingers and back to the pool. “It is all but impossible to retrieve such structures intact. They are like sculptures of dry sand.”

Ted straightened up painfully; it felt as if there was no blood at all in his lower legs. “Structures?”

“Yes.” Blue shifted his position, looking for more samples. “From the aerial shots and samples taken on the ground, it appears that the Moonseed is endeavouring to construct something here. A kind of dish, with a parabolic profile, half a mile across—”

“Covering most of Arthur’s Seat, then.”

“Yes.”

“This Moonseed is a rock-eating germ. How can it construct anything?”

“It does not eat rock,” Blue said, “and neither is it a bug. It moves atomic particles, sometimes molecules, to build structures from the subatomic level up. As far as we can tell, these structures are perfect. Without defect.”

“Not so perfect. That thing in your hands just fell apart.”

“It’s true we can disrupt the structures if we catch them early enough. But I don’t think whatever the Moonseed is building here is meant for this world.”

“What do you mean?”

But Blue wouldn’t answer.

He bent again, trying to collect more samples of the Moonseed structure, which he preserved in clear, fast-setting plastic.

When they were done, they walked gingerly back along the neck of land.

Ted pointed to the Cathedral on Castle Rock. “We go there.”

Blue looked at him curiously, face masked by the layers of dust and dirt on his faceplate. “Why?”

“Something I’m looking for.”

Blue shouldered his equipment. “Henry told me about your son.”

“In between porking my daughter.”

“I understand why you have come. I feel a certain — responsibility — for bringing you here.” He looked into Ted’s face. “So I feel I have to tell you this. Smell the coffee, Ted. Your son is dead. I saw him here, just before the great eruption. He is lost, here in the ruins of Edinburgh. At best you will only find his body. Probably not even that.”

“I know,” Ted said softly. “I’ve known that from the beginning.”

“Then what are you looking for?”

Ted said, “Are you coming with me, or do I go alone?”

Blue sighed. “If I abandon you, Henry will squeal like a pig stuck under a gate. Come.”

With Blue leading, they worked steadily over the shattered cityscape towards the building.

St Giles” was a great sandstone block, atypically low and squat for a Gothic cathedral, but that had evidently helped it survive; the pyroclastic flow, washing over Castle Rock, had heaped up against the eastern wall, but not breached it. Still, the stained glass windows had shattered or, it looked like, melted; and the ornate crowned tower, the Scottish equivalent of a spire, was gone.

They paused for breath.

“St Giles’,” Ted said. “Patron saint of cripples, lepers and tramps.”

“Very appropriate,” Blue said. “I am impressed it has survived at all.”

Ted pointed. “Those pillars holding up the tower are nine hundred years old. Even survived the English burning the bloody place down. They’ll last a wee while yet. Come on.”

The big wooden doors of the Cathedral had been smashed in, the shards burned. Ted and Blue picked their way over the wreckage, the scorched wood crunching under their thick-soled boots.

The roof was destroyed — debris was scattered over the aisles and altar and the rows of pews — and silvery, alien daylight streamed into the dusty interior through the gaping roof and the empty window frames. Ted stood in the doorway for a few minutes, letting his gaze follow the soothing geometry of sunbeams. As if one part of the world still worked. The Cathedral was full of light, in fact, probably brighter than it had been since the day the roof was put on. The uniform grey and black was oddly pleasing, like a charcoal sketch.

He moved forward. He had to push through the ash layers, climb over the cold lava bombs which lay beneath it, like pushing through a shallow stream.

There were people in the pews, he saw.

Some were sitting, some had been kneeling, some seemed to have fallen. Their bodies were barely visible, all but drowned by the ash. Here was a woman — he couldn’t tell her age — her face tipped up to the ceiling, he supposed towards God, her mouth open and clogged with ash.

“The roof probably gave way immediately,” Blue said gently. “The ceiling rubble came down on them, and then pumice, hot ash, steam, gases. They must have died very quickly. Probably of suffocation.”

“I suppose they came here for shelter.”

“I suppose so. Perhaps we will find the priest at his altar. If they are undisturbed, perhaps this will form another Pompeii, for future archaeologists.”

Ted stopped beside another woman. “They are not undisturbed.”

Blue bent to see.

A necklace had been ripped from the woman’s neck. There were fingermarks, cut deep in the layers of ash. “He’s here,” Ted said.

It didn’t take long to find him. There weren’t many places left in the Cathedral intact enough to hide in.

He was in the Thistle Chapel, an ornate, heavily ornamented twentieth-century annexe of the Cathedral. The windows had blown in, but its roof had survived, and so had most of the Chapel’s ornamentation: carved animals, angels playing musical instruments, including bagpipes.