We disentangled ourselves, cursing, and sat facing each other, slumped against opposite walls of the new tunnel we’d ended up in. I aimed the torch up and saw the edges of the hole we’d dived through, and then looked around us. We’d fallen another three metres deeper into the catacombs, tumbling into a short linking passage no more than a dozen metres long. I crawled to the far end of it. Back in the direction we’d come from – beneath our old tunnel – was a seventy-metre straight section that ended in a T-junction. To the right the passageway carried on for what looked like more than two hundred metres. There was no sign in either direction of the collapse on the level above. Once I was sure that we were alone, I turned the torch on the young man. He put his hands up to shield his face from the brightness and I adjusted the barrel of the M4 so that the muzzle wasn’t pointing at him or the torch blinding him.

‘Ça va?’ I asked him, though he didn’t look OK. His clothes were sodden; the fine dust from the crumbling cave had stuck to him and turned into a milky white paste that coated his face, jacket, boots; and his hands were trembling and bloodied, his elbows and knees grazed and bleeding. But his breathing was calm, and his eyes were focused. I’d been clobbered myself, but neither of us had been badly injured.

‘Oui,’ he replied. ‘Qu’est ce …’ His French faltered. ‘What happened?’ he tried again in English. His accent was soft, educated, the vowels stretched and flattened in turn. Indian, maybe. Or Pakistani. Via England.

‘Two men tried to drown you,’ I replied. ‘I killed them and then the roof collapsed.’

He opened his mouth to speak, thought better of it, but then asked, anyway.

‘Are you a cop?’

‘No,’ I said. ‘I’m not.’ He stared involuntarily at the rifle in my hand. ‘It’s complicated,’ I said. Which was true. ‘I’m not going to hurt you. But we’re in deep shit.’

‘Really?’ he said, closing his eyes as he rubbed at the grime covering his face. His tone was sarcastic. But his hands were still shaking.

‘“Really” I’m not going to hurt you, or “really” we’re in deep shit?’

‘Really …’ He screwed up his eyes harder, trying not to cry. ‘Really, just … who are you?’ His tone was plaintive now. The self-defence of sarcasm was too much effort to maintain.