It was the chute down which the cemetery above had been emptied into the limestone cavern below. Like a macabre calcified waterfall, the remnants of the long dead spewed out from its mouth, engulfing the room with the mortal remains of generation upon generation of Parisians. At first the bones had been sorted and stacked, filling the charnel house with concentric rings of partial skeletons. At the end, though, it looked like the last deposits of upwards of a thousand bodies must have just been tipped into the gallery below.

I unslung the M4, stuck the pistol back into my waistband and then scrambled up the bone scree to investigate. Skulls click-clacked like castanets. I squirmed around on to my back and found myself taking a deep breath as I pushed my head up into the hole. I could see nothing.

‘Baaz, are you sure this goes through? It’s pitch black. Not even a speck of sunshine.’

‘Look at your watch,’ he replied. I wriggled out and did a double take. It was seven o’clock in the evening. We’d been walking for more than five hours. ‘Everyone loses track of time down here,’ he said with a smile. ‘It’s unreal.’

I was still on my back, suspended by that massive bed of bones, staring at the ceiling.

‘How do you do it?’ I asked him. ‘Navigate, I mean. No maps. No waypoints.’ I pushed myself up on to my elbows and looked down at him. ‘It’s not normal,’ I said. The Glock pressed into my spine. It’s not possible, I thought.

He looked up at me and cocked his head to one side.

‘My brain,’ he said. ‘It’s different. It works differently.’

‘Different how? To what?’

‘To yours. To everyone’s.’ He smiled again. ‘Chust chalak, my mother says. He put on a strong Punjabi accent, mimicking her pronunciation. ‘Clever clogs. I remember things. Patterns. Numbers. Once I’ve seen them, I can’t forget them.’ He tapped his right temple. ‘It’s bloody crowded in there.’

I looked back into the hole.

‘Well, it’s going to be bloody crowded up there, too.’ I waved my right hand. ‘Pass me the torch, will you?’

He did. I wanted to stay dark during the climb, for when we emerged. But I needed to see what I was getting into first. I took the black barrel of the LED from Baaz’s fingers and angled the beam into the shaft so that as little glare as possible would leak out. Immediately above my face the outline of a skull leapt out, mouth grimacing in the slanting torchlight. I swept the beam out at first by a foot, and then another. As the light climbed higher, I understood more clearly the true horror ahead. No wonder no one used this exit. Even if the cops did know about it, no one would ever be stupid – or desperate – enough to try it.