‘Excellent,’ Vale said. ‘Ah, we are almost there. Strong-rock, once we cross that open area, we will come to that final lake I mentioned. There is a last high bridge across it, which will take us almost to the exit. They might well have armed guards posted there - it’s what I would do. You may need to rouse the water to wash them away first. Do you think you can do it?’ He turned to meet Kai’s eyes.

‘It will be my pleasure,’ Kai said. ‘But this place is coming apart. What if I end up damaging the exit?’

‘It’s still our best option,’ Irene said firmly, deciding to keep her worries to herself. ‘Besides, have you noticed that the earth-tremors have slacked off a little? Maybe their purpose was to drive us into the ambush—’

More rubble fell - nearly on top of them this time - and the marble paving shivered under their feet, cracking into fragments. A huge gust of wind made them all stagger.

‘Or maybe not. Let’s get this done fast,’ Irene said hastily. She didn’t add Because it may be listening, but she could see it in their faces.

They moved forward together at a run. Caution was pointless now, for the crashing stone and the wind drowned out any noise they made - and there was no sign of guards or snipers as they mounted the last bridge.

Almost no sign. Irene caught a flash of red, a sleeve hastily pulled back into cover, violently obvious against the colourless marble.

‘You think it’s clear?’ Kai asked, his voice pitched just loud enough to be heard.

‘No, look there,’ Irene said, pointing, her voice equally low. ‘They’ll be sitting on top of the entrance, just waiting for us to come to them.’

‘It was all too likely,’ Vale agreed. ‘Now, as we planned, Strongrock.’

Kai nodded. Leaving wet footprints behind, he sprinted to the edge of the bridge and threw himself into the lake below in a running dive, vanishing beneath the surface. The water began to swell into a growing wave, sweeping forward and upwards, growing higher with every moment.

The arched length of bridge shook beneath them.

If they went forward, they might be running into an ambush. If they stayed where they were, she and Vale could end up crushed. ‘Stone, hold together!’ Irene shouted as loudly as she could. Her voice wouldn’t carry to the ceiling, but if it could just keep the bridge together for long enough, Kai could deal with the soldiers.

Screams carried over the noise of falling rocks from ahead of them. Beneath Irene’s feet a long crack ran through the bridge, black against the white marble, tracing across it like a child’s scribble. The bridge groaned underneath them in a long roar of fracturing stone, but it stayed in one piece.

Irene and Vale exchanged a glance, then decided that an ambush was the lesser danger. The marble paving was a heaving surface under them as they ran, still holding together but trembling against the forces threatening to shake it apart. It was barely possible to hear anything now, above the shuddering tumble of stone and the screaming wind.

‘Over here!’ Kai roared in a voice almost too loud for human lungs. ‘I’ve cleared our path!’

There was a clear ping as something rang against the marble rail beside Irene. At first she thought it was a fragment of stone, then she recognized it as a bullet. ‘Oh no, he hasn’t,’ she muttered.

‘It may be the best he can do,’ Vale shouted through the wind. He had paused at the sound of the bullet, like her, and was looking around desperately. ‘Winters, there’s no other way out of here, we must risk it. Come on!’

It was quite true. But a Librarian couldn’t speak fast enough to stop a bullet. They were about halfway across, so the considerable remaining length of bridge was downhill, but that wasn’t much help …

Oh yes, it was. ‘Kai, get ready to catch us!’ she screamed. ‘Marble bridge surface, become a hundred times more slippery!’

She launched herself into a desperate skid, and the next tremor tumbled her onto her rear. That only speeded her velocity. Like a child coasting down a hill, she skimmed helplessly and unstoppably down the curve of the bridge, far quicker than if she’d been running. The shuddering stone also pitched her unpredictably from side to side. She hoped that would foil the snipers as she shot forward. From the curses behind her, Vale was just as unable to control his motion, and a scatter of bullets cracked against the stone a few yards behind them.

Kai was standing on the now-ruffled surface of the sunken reservoir, where the end of the bridge met the paving. And he was surrounded by a moving coil of water, which had formed itself into a shield around him. Half a dozen guards were strewn unconscious on the ground, or groaning in the aftermath of being hit by the giant wave. No doubt their gunpowder was as soaked as they were. And twenty yards further on their goal was in sight at last: the metal stairway that had brought them here from Venice, standing upright within the yawning dark chasm that became the Campanile. The metal bridge that spanned that abyss to join the paving lay before them. Kai nodded as he caught sight of them, his posture braced and ready, and as Irene and Vale came skidding towards him, he flung his arms high in the air.

The water came shooting upwards around him, encasing him in a rising pillar of water. Like an unnatural tornado, the water moved upwards towards the far-distant ceiling with a roar that was audible even against the falling stone. Then it stopped, its power harnessed on the edge of breaking. Within its grip, Kai’s hair floated around his head as if blown back by a wind, and the sleeves of his shirt rippled from the strength of the flow.