‘Who built this?’ he asked the man in front of him. It was Colby, who was looking around in wonder, his mouth open.

‘I don’t know,’ he said. ‘I never even knew something like this existed.’

They walked along the path, spiralling round and round the wall, until it led them into the rock itself through a low tunnel, then another, till they came to a large, curved—almost round—cave. It was almost half a mile to the far wall. More a vast elevated hollow space within the rock than a cave, it had no ceiling to speak of, just countless folds of rock at different heights above their heads. On their left the stony floor ended in a sudden steep edge—nothingness beyond all the way as it curved back under their feet to meet the wall straight ahead of them. And there, in the shadows of that wall, was another opening that Marcus was pointing at.

Albert walked to the edge of the curved floor, gazing down in a mixture of wonder and bewilderment. Below them was another clearing, so huge it dwarfed the one they were standing in, separated from them by a sheer cliff of rock a good twenty feet down. Much like the floor under their feet, it was made of flattened rock, except the surface was pockmarked with dozens and dozens of large holes, each at least three feet in diameter, so it looked like a gargantuan rocky pincushion with the pins missing.

As he stood there, the others came to join him—first Theo and June, then Colby and his men—and they all stood there, looking down at the strange clearing. Marcus came up to stand next to them. Albert turned to him curiously. ‘What’s that?’ he asked.

‘Horrors,’ said Marcus.

‘What’s that mean?’ said Albert, frowning down at the strange surface. Colby and the other outliers seemed almost transfixed by it.

‘Perhaps you would be wiser not to find ou—’ he broke off, whirling to look up towards the ledges and crevices in the rock way overhead. ‘We are not alone.’

And then Albert saw them—several men, almost a dozen, moving swiftly along the ledges, making their way down towards them on ropes that now hung from high above. How had he not seen them earlier?

On his left, he heard Colby swear. ‘Renegades!’ he hissed.

They heard footsteps—sudden, urgent footsteps—echoing loudly round the cavern and swung around to see three men racing towards them, holding long knives.

‘We got ’em!’ yelled one delightedly, pulling up about six feet short of them. He was big and bulky, with a pug nose and a remarkably squeaky voice. It was even higher than Theo’s. ‘They ain’t got nowhere to go.’

While he made his report, his companions raced ahead, leaping forward to attack Marcus, who was closest to them.