“Brock Marsh sewers, for example. All the unstable runoff from all those labs and experiments, accumulating over the years . . . makes for a very unpredictable population of vermin. Rats the size of pigs, speaking in tongues. Blind pygmy crocodiles, whose great-great-great-grandparents escaped from the zoo. Crossbreeds of all sorts.

“Over in Gross Coil and Skulkford the city’s sitting on layers of older buildings. For hundreds of years they sunk into the mire, and they’d just build new ones on top of them. The pavement’s only been solid there for a hundred and fifty years. Over there, the sewers feed into old basements and bedrooms. The tunnels like this one lead into submerged streets. You can still see the road-names. Rotten houses under a brick sky. Straight up. The shit flows along channels and then through windows and doors.

“That’s where the undergangs live. They used to be human, or their parents did, but they’ve spent too long down here. They aren’t a pretty sight.”

He hawked and spat noisily into the slow ooze.

“Still. Rather the undergangs than the ghuls. Or the trow.” He laughed, but it was without any humour. Yagharek could not tell if Lemuel was mocking him.

Lemuel lapsed into silence. For some minutes, there was no sound except the slosh of their legs through the thick effluvia. Then Yagharek heard voices. He stiffened and gripped Lemuel’s shirt, but a moment later he heard them clearly, and they were Isaac’s and Derkhan’s.

The excremental water seemed to carry light with it, from around a corner.

Bent-backed and swearing with effort, Yagharek and Lemuel wound through the twisting brick junctions and turned into the little room under Murkside’s heart.

Isaac and Derkhan were yelling at each other. Isaac saw Yagharek and Lemuel over Derkhan’s shoulder. He raised his arms to them.

“Dammit, there you are!” He strode past Derkhan towards them. Yagharek held out a bag of food at him. Isaac ignored it. “Lem, Yag,” he said urgently. “We have to move fast.”

“Now hold on . . .” began Lemuel, but Isaac ignored him.

“Listen, dammit,” Isaac shouted. “The construct’s talked to me!”

Lemuel’s mouth stayed open, but he was silent. No one spoke for a moment.

“All right?” said Isaac. “It’s intelligent, dammit, it’s sentient . . . something’s happened in its head. The rumours about CI are true! Some virus, some programme glitch . . . And although it won’t come out and say it, I think it’s hinted that that damned repairman may have given it a helping hand along the way. And the upshot is the damn thing can think. It’s seen everything! It was there when the slake-moth took Lublamai. It . . .”

“Hold on!” shouted Lemuel. “It spoke to you?”